I'm officially declaring these mixed berry crumble bars the dessert of summer! With a sweet, jammy fruit filling sandwiched between a buttery brown sugar oat crust and crumble topping, they're easy to make and still a total crowd-pleaser. I like to use a mix of strawberries, blueberries, raspberries, and blackberries, but I've made this recipe with all different combinations of berries - including fresh and frozen berries - and it turns out perfectly every time.
If you invite me to a BBQ this summer, you can be confident that I'll be bringing along a batch of these mixed berry brown sugar crumble bars!
These simple-to-make bars are perfect for every occasion, from school bake sales to summer picnics to weeknight dessert. I may have even eaten one for breakfast with no regrets! Basically I'm just looking for any excuse to make (and eat!) them.
It's the combination of golden, buttery brown sugar oat crumbles and sweet, juicy fruit that I can't get enough of! These bars have that same cozy, nostalgic feeling as a bowl of classic fruit crumble but in a portable, easy-to-slice bar form that's so much simpler to serve.
Why You'll Love This Recipe
My favorite kind of dessert to make is one that's easy enough to make on a weeknight but impressive enough to serve at a party, and these mixed berry oatmeal crumble bars are exactly that. Here's why you'll love this recipe:
- Easy to Make - The brown sugar oat crumble pulls double duty as both the sturdy, golden base and the irresistible crumble topping. It comes together in just one bowl with a handful of simple pantry staples.
- Works with Fresh and Frozen Berries - Have some fresh berries you need to use up? Perfect! Have a bag of frozen berries sitting in your freezer (or want to save some money buying berries for this recipe)? Frozen works great, too!
- Thick, Jammy Fruit Filling - This recipe uses the same pre-cooked filling method from my blueberry pie filling recipe to make sure the berry filling is nice and thick before it ever hits the crust. A few minutes on the stovetop makes all the difference, and it's the secret to a fruit filling that sets and bars with clean slices every single time.
- Crowd-Pleasing - With their sweet fruity flavor, and lightly chewy brown sugar oatmeal crust, everyone loves these bars! Your friends and family will ask you for the recipe, so be ready. 😉
Ingredient Notes & Substitutions
See recipe card at the bottom of this post for full ingredient list and measurements.
Berries - Any berry works here! Blueberries, raspberries, blackberries, strawberries, or a mix of all of them (my favorite!). Fresh or frozen berries both work beautifully. If using frozen berries, there's no need to thaw first, so just add them straight to the pan.
Cornstarch - This is what thickens the berry filling so it sets properly and gives you clean slices. Don't skip it!
Lemon Juice - A little bit of lemon juice helps to brighten the flavor of the fruit filling. Fresh lemon juice tastes the best, but concentrate will work in a pinch.
Old-Fashioned Oats - Be sure to use old-fashioned rolled oats here, not quick oats. Quick oats will absorb too much moisture from the butter and you'll miss out on the chewy texture that old-fashioned rolled oats gives us.
Brown Sugar - Light brown sugar gives the crumble its rich, lightly caramel-y flavor. It also helps the crust and topping get those beautiful golden edges.
Melted Butter - Using melted butter (rather than cold butter) makes this crumble come together so easily. Just toss it in with a fork and the dough forms almost instantly.
Special Equipment
In addition to essential baking equipment (like a mixing bowl and spatula), I recommend some special tools for making this recipe.
- A sauté pan, for pre-cooking the berry filling.
- An 8x8 metal baking pan, for baking the bars.
Step-By-Step Recipe Instructions
See recipe card at the bottom of this post for detailed recipe instructions.
Step 1: Prep the fruit filling. Combine berries, sugar, cornstarch, lemon juice, and salt. Stir to coat the berries.
Step 2: Cook the filling on the stove until the berries release their juices. Continue to cook until the filling thickens and begins to bubble. Let the filling cook while bubbling for 1 minute. Remove the pan from the heat and let the filling cool slightly.
Step 3: Make the crumble dough. Whisk to combine oats, flour, brown sugar, baking powder, and salt.
Step 4: Add melted butter and toss gently with a fork until a dough forms.
Step 5: Add ⅔ of the crumble dough to an 8x8 metal pan lined with parchment paper. Gently press the dough into the pan to form a crust. Parbake the crust in the oven preheated to 350°F for 10 minutes, until golden.
Step 6: Spread the berry filling over the crust. Crumble the remaining dough over the berry filling.
Step 7: Bake for another 20-25 minutes, or until the filling is bubbling and the crumbles on top are golden. Let cool before slicing and serving.
Allison's Top Tips
I have no self control around these bars (ha!), so I like to cut them nice and big, getting just 9 bars from an 8x8 pan. However, you can cut this recipe into 16 squares if you prefer to serve smaller portions.
Want to double this recipe to serve to a crowd? Bake it in a 9x13 pan and add about 5 minutes to the bake time.
For an extra special treat, serve these bars slightly warm with a scoop of vanilla ice cream!
Storage Instructions
These bars stay fresh for days, so they're a great make-ahead option if you need to bring a dessert to a party! Once completely cooled, seal the bars in an airtight container and store them in the fridge for up to 5 days.
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Recipe
Mixed Berry Crumble Bars
Special Equipment
- Saute pan
- 8x8 metal baking pan
Ingredients
Berry Filling
- 12 ounces (340 grams) fresh or frozen berries (if frozen, do not thaw)
- ¼ cup (49 grams) granulated sugar
- 1 tablespoon (7 grams) cornstarch
- 2 teaspoons (9 grams) lemon juice
- 1 pinch fine sea salt
Brown Sugar Oat Crumble
- 1 ½ cups (134 grams) old-fashioned oats
- 1 ½ cups (180 grams) all-purpose flour
- 1 cup (213 grams) light brown sugar
- ½ teaspoon (2 grams) baking powder
- ½ teaspoon (3 grams) fine sea salt
- ¾ cup (170 grams) unsalted butter melted
Instructions
- Preheat the oven to 350°F. Line an 8x8 metal baking pan with parchment paper. Set aside.
Prep the fruit filling.
- In a medium pan, combine berries, sugar, cornstarch, lemon juice, and salt. Stir to coat the berries.12 ounces (340 grams) fresh or frozen berries, ¼ cup (49 grams) granulated sugar, 1 tablespoon (7 grams) cornstarch, 2 teaspoons (9 grams) lemon juice, 1 pinch fine sea salt
- Cook the filling on the stove on medium-low heat until the berries release their juices, about 10 minutes. Stir the mixture frequently while cooking.
- Turn the heat up to medium and continue to cook, stirring frequently, until the filling thickens and begins to bubble, about 5 more minutes. Let the filling cook while bubbling for 1 minute.
- Remove the pan from the heat and let the filling cool slightly while you make the crumble dough.
Make the crumble dough
- In a large bowl, add oats, flour, brown sugar, baking powder, and salt. Whisk to combine.1 ½ cups (134 grams) old-fashioned oats, 1 ½ cups (180 grams) all-purpose flour, 1 cup (213 grams) light brown sugar, ½ teaspoon (2 grams) baking powder, ½ teaspoon (3 grams) fine sea salt
- Add melted butter and toss gently with a fork until a dough forms.¾ cup (170 grams) unsalted butter
Assemble and bake the bars.
- Add ⅔ of the crumble dough to the prepared pan. Use your hands to gently press the dough into the pan to form a crust that reaches the edges of the pan.
- Parbake the crust in the oven for 10 minutes or until golden. Remove from the oven and let sit for 2 minutes.
- Carefully spread the berry filling over the crust.
- Use your fingers to crumble the remaining dough over the berry filling.
- Return the pan to the oven and bake for another 20-25 minutes, or until the filling is bubbling and the crumbles on top are golden.
- Let cool before slicing and serving.
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