Keto Strawberry Rhubarb Upside Down Cake
Published May 9, 2020 • Updated February 28, 2026
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I've made this strawberry rhubarb cake more times than I can count, and the moment you flip that bundt pan and see the caramelized fruit layer is worth every bit of effort. Moist almond flour sponge with a low carb caramel topping that stays soft and pulls away from the pan clean.
Strawberry rhubarb is one of those pairings I grew up eating in pies and cobblers every summer, and when I went keto back in 2012, I thought that combo was gone for good. It took me a while, but I figured out how to get that same tart-sweet balance into an upside-down bundt cake that actually releases from the pan without leaving half the topping behind.
The secret is the caramel layer. I use golden monk fruit whisked with butter until it bubbles, then pour it into the bottom of a greased bundt pan before spooning the fruit mixture on top. The golden monk fruit matters here. I tried this with straight erythritol early on and the caramel re-crystallized as the cake cooled, basically gluing the fruit to the pan. The monk fruit blend stays soft and pliable, which is why the topping releases in one piece when you flip.
The cake itself is an almond flour and coconut flour blend with sour cream for moisture and lemon zest for brightness. I whip the butter for a full 3-4 minutes before adding anything else because that air is what gives the crumb its lift (skip this and you get a dense puck). The batter goes on top of the fruit layer, and the whole thing bakes at 325 for about 45 minutes. Lower and slower than most cakes, but almond flour burns fast at higher temps and I learned that the hard way.
Here is what I wish someone had told me the first time: let the cake cool in the pan for a full 30 minutes before flipping. Not 10, not 15. The almond flour crumb needs that time to set, and the caramel needs to firm up just enough to hold the fruit in place without hardening. If you flip too early, the caramel runs everywhere. Too late and it sticks. Thirty minutes is the sweet spot I landed on after testing it more times than I want to admit.
One thing that surprised me: frozen rhubarb actually works better than fresh for the topping layer. A reader named Taylor tried it in February when fresh rhubarb was nowhere to be found, and the frozen stalks released more liquid during baking, creating a thicker, more concentrated jammy layer at the bottom of the bundt. I tested it myself after reading her comment and she was right. The caramelization was deeper and more intense than what I get with fresh. So if you are making this outside of summer, grab frozen and do not bother thawing it first.
I love serving this with a scoop of whipped cream keto strawberry shortcake style, or alongside strawberry shortcake kebabs for a full spread. If you want a simpler keto strawberry cake without the upside-down drama, I have that too. And for birthdays, Jeane made this for her husband’s 71st, accidentally added the reserved butter to the batter instead of the pan, compensated with extra sour cream and a fresh butter batch, and it still turned out beautiful. That is how forgiving this recipe is. If you are into low carb baking with almond flour, my keto coffee cake uses a similar base.
Ingredients
4 large strawberries, cut into small chunks -
3 stalks rhubarb, chopped
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2 teaspoons arrowroot powder
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3/4 cup erythritol or monk fruit sweetener, divided
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1 1/4 cup of unsalted butter, softened
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1/3 cup golden monk fruit
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1 1/2 cups almond flour
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1/4 cup coconut flour
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2 teaspoons baking powder
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1/2 teaspoon salt
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zest of one lemon
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1 teaspoon vanilla
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4 eggs
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1/3 cup sour cream or low-carb yogurt
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2 teaspoons lemon juice
Step by Step Instructions
Step by Step Instructions
Get a medium bowl
In a medium bowl, mix together rhubarb, strawberries, arrowroot powder, and the 1/4 cup erythritol or monk fruit sweetener. Set aside.
Get a large bowl
In a large bowl, whip butter with an electric mixer until light in color and fluffy (about 3-4 minutes of mixing).
Mix until fluffy
Add 1/2 cup erythritol or monk fruit sweetener and lemon zest continue mixing until fluffy. Then take out 1/4 cup of the butter mixture to use for later.
Add eggs
Add eggs, mixing in one at a time. Mix in sour cream, lemon juice and vanilla unti combined.
In a saucepan
In a saucepan over medium heat, add the 1/4 cup of reserved butter with the golden monk fruit. Whisk until bubbly (about 2-3 minutes). Remove from heat. Pour in the bottom of the bundt pan.
Bake it
Scoop cake batter on top of fruit layer and spread out evenly. Bake at 325 for about 45 minutes or until cake is firm to the touch.
Nutrition disclaimer
The nutrition information provided is an estimate and is for informational purposes only. I am a Doctor of Pharmacy (Pharm.D.); however, this content is not intended to be a substitute for professional medical advice, diagnosis, or treatment. Always consult your physician or other qualified health provider before making any lifestyle changes or beginning a new nutrition program.
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Get My Macros + Recipes →Frequently Asked Questions
Why does my upside-down cake stick to the bundt pan, and how do I prevent it?
I dealt with this myself until I figured out the pan prep. Butter leaves milk solids behind that basically weld the caramel to the pan as it cools. I switched to shortening or Baker's Joy spray and the sticking stopped. Grease every ridge and curve, not just the bottom. If it does stick after baking, lay a warm damp towel over the inverted pan for 2-3 minutes. The steam loosens the caramel layer enough to release. Reader Melanie had this problem twice before switching her spray method, so you are not alone.
Should I use fresh or frozen rhubarb for this recipe?
I have used both, and I prefer frozen now. A reader named Taylor used frozen rhubarb in February and discovered it produces a thicker, more concentrated jammy layer than fresh. I tested it after reading her comment and she was right. The extra moisture from the frozen stalks caramelizes into something deeper and more intense. Do not thaw it first. Just chop and toss it in with the strawberries.
Can I use coconut oil instead of butter in this cake?
I have not done a full side-by-side test on this one, but solid coconut oil should work as a 1:1 swap in the batter. The texture will be slightly different and you will pick up a mild coconut flavor. For the caramel layer, I would be more cautious since butter gives it that rich golden color and the milk fat helps it bubble properly. If you try coconut oil for the caramel, whisk it on slightly lower heat and watch it closely.
What is the best sweetener for the caramel layer?
I use golden monk fruit and it is the one part of this recipe where my sweetener choice really matters. Golden monk fruit (or an allulose blend) stays soft and pliable as the cake cools, which is why the topping releases cleanly when you flip. I tried pure erythritol early on and it re-crystallized into a hard shell that fused the fruit to the pan. If sticking has been your problem, switch your caramel sweetener before you blame the pan.
Can I bake this in a tube pan or round pan instead of a bundt?
Yes, and a tube pan makes the flip easier in my experience. Reader Christine shared a great trick: fold a round parchment sheet into a cone shape, snip the tip to fit over the center tube, and spray everything including the parchment with baking spray. The tube pan also lets you run a knife along the sides to loosen before flipping, which you cannot do with a bundt. I still prefer the bundt for the look, but if clean release is your priority, a tube pan with parchment is the safer bet. If you are new to almond flour baking, the tube pan is more forgiving.
How long should I wait before flipping the cake?
Thirty minutes. I tested shorter and longer cool times and this is the window that works. Flip too early and the caramel runs because it has not set. Wait too long and the sugar firms up and bonds to the pan. I pull the cake from the oven, set a timer for 30 minutes, loosen the edges gently, then flip onto a plate in one confident motion. Do not peek or wiggle it during the cool time.
Can I make this keto rhubarb cake ahead of time?
I have made it the night before and it holds up well. Flip it onto the serving plate, let it cool completely, then cover with plastic wrap and refrigerate. The caramel layer stays intact in the fridge for up to 5 days. I would not freeze it already flipped because the fruit layer gets watery when it thaws. If you want to freeze, keep it in the pan unflipped, wrap tightly, and freeze for up to 3 months. Flip after thawing in the fridge overnight. I have done this for a party alongside a keto birthday cake and both held up perfectly.
What if I accidentally add the reserved butter to the batter?
You are in good company. Jeane did exactly this when making the cake for her husband's 71st birthday. She set the sour cream and reserved butter side by side and grabbed the wrong one. Her fix was adding the sour cream too (since it was going in anyway) and mixing up a fresh batch of sweetened butter for the pan. The cake baked perfectly in 45 minutes and only a few pieces of topping stuck. My recipe is forgiving with extra fat. Just make fresh caramel butter and keep going.
When strawberries and rhubarb are in season, something magical happens. They combine creating a unique flavor combination – tart and sweet. Strawberries and rhubarb are in season at the same time, so it’s only natural that I create some tasty dessert recipes using them.
This keto strawberry rhubarb cake is a sugar free upside down cake. The cake is spongy with a hint of lemon flavor. The crumb is moist from the strawberry rhubarb topping that drips over it when you flip it upside down.
Rhubarb is a vegetable that is low in carbs. It has 1.2 net carbs per stalk. Rhubarb works great on keto.
It has a sour, bitter taste but when you contrast it with a sweet fruit like strawberries, it’s delicious!
Most berries are lower in sugar. Strawberries work well on a low carb diet since they are lower in natural sugars. They have 8.7 net carbs per cup of chopped berries.
Strawberries are in season beginning mid May to late June, so take advantage of strawberry season and make your favorite strawberry desserts.
This is an upside down cake recipe. You can use a regular cake pan if you want to, but I thought using a bundt pan was prettier – especially because I like to make this cake late Spring when strawberries and rhubarb are in season, so around Mother’s Day.
The size of bundt pan I use is a 10 inch pan. You can use smaller if you want. Remember to spray coconut or avocado oil on the bundt pan before adding the batter so it slides out easily when you flip it upside down.
The directions for this sugar free cake state to whip the butter using an electric mixer until the butter is light in color and fluffy. I recommend doing this step because it creates air and volume in the butter which gives you a lighter, spongier cake.
Beat the butter with the mixer until it is fluffy and the color has changed from yellow to pale yellow or almost white. This step takes several minutes but the end result is worth it!
Added a teaspoon of fresh grated ginger to the fruit layer and rhubarb and ginger are just made for each other. That little warm kick under the caramel sweetness caught me off guard, and now I can't go back.
Hadn't thought of ginger in the fruit layer but the more I think about it the more it tracks. That warmth under the caramel is going to hit differently. Stealing this.
I've made basically every keto dessert and had real doubts about rhubarb, it always goes so sour on me even in regular baking. Had some sitting in the fridge and just went for it. That moment when you flip the bundt pan and the caramelized fruit comes out clean in one piece, I genuinely didn't expect it to look that good. The golden monk fruit does something to the rhubarb, no tartness at all, just this jammy layer that beats anything else I've tried on keto.
That flip gets me every time. And the monk fruit thing with high-acid fruit is something I didn't expect the first time either. The tartness just disappears into that layer.
Fourth time making this and the one change that finally nailed it: an extra stalk of rhubarb in the fruit layer, which adds just enough tartness to keep the caramel from going too sweet.
Yeah, the caramel can run sweet, especially with golden monk fruit. An extra rhubarb stalk is a clean fix. Trying four next time.
Grabbed frozen rhubarb because fresh wasn't happening in February and it caramelized into this thick jammy layer at the bottom of the bundt (way more concentrated than I expected), not using fresh for this one ever.
Frozen pulls more moisture and it all cooks off right into that layer. I tested it after you mentioned it and yeah, not going back to fresh for this one.
Annie, thanks for a delicious recipe! I made it for my husband's 71st birthday & he loved it! I flubbed by adding the reserved sweetened butter to the batter (pro tip: don't sit the sour cream & butter side-by-side). I went ahead & put in the sour cream too, praying it would turn out ok & mixed up more sweetened butter. It baked in 45 min, cooled in the pan for 30 min. When I flipped it over, only a few pieces of topping were left behind in the pan. Yum!
Ha, I've done that exact thing. The double butter probably made it extra rich honestly. And if only a few pieces stuck, that's a win. Mine sometimes leaves half the topping behind if I flip too early.
Can I use softened solid coconut oil instead of butter?
I haven't tried that substitution. I think it would work. If you try it, let me know how it goes!
I have made this twice and it is absolutely delicious. However both times I have cooled the cake for a min of 45 minutes to an hour. Each time I have had the issue of the fruit/veg (I used the 1 tsp xanthan gum swap) top sticking and staying in the bundt pan, the first time taking a piece of the cake too as I flipped it. I liberally sprayed the pan with coconut oil before baking, and the second I also used some butter with no difference. Any tips or idea's as to why this is happening? If it's for me I don't mind, but this is for a family member that cannot eat the traditional pineapple upside down, so I would like to present it as perfectly as possible.
Thanks,
Mel
Try round parchment sheets and use a tube pan instead of a bundt pan. If you fold the round in half to form a semi-circle, then fold 2 more times to get a cone shape, you then cut the tip about an inch up to fit over the tube in the middle of the pan. Then spray everything including the parchment with a good baking spray and it should peel off nicely once the cake has cooled. Of course, the benefit of using a tube pan also is that you can use a knife to loosen the sides before flipping. Hope this helps, and good luck.