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Mixed Berry Crumble Bars Recipe — Quick Fruit Dessert Bars

Buttery oatmeal crumbles and sweet, juicy berries make these Mixed Berry Crumble Bars impossible to resist. The recipe uses the same oat mixture for the base and the crumble topping, with a bright, juicy middle of strawberries, blueberries, raspberries and blackberries that tastes like summer. 3 berry crumble bars made with oatmeal and brown sugar stacked on a white plate.

These mixed berry crumble bars feature a soft brown-sugar oatmeal base, a juicy berry filling, and a cinnamon-scented crumble topping. They have the familiar comfort of a fruit crumble but are easier to transport and offer more oatmeal-to-berry ratio—perfect for breakfasts, potlucks, or a simple dessert. You can make them with fresh or frozen berries.

There are a number of crumble-bar variations on Just So Tasty, and it’s helpful to adapt the filling slightly depending on the berries you use. Different berries release different amounts of juice, so adjusting the lemon and cornstarch prevents an overly soggy bar.

Berry Crumble Bars Overhead shot of mixed berry oatmeal crumble bars fresh out of the oven.

This recipe makes a large batch baked in a 9×13-inch pan. If you prefer a smaller batch, halve the ingredients and bake in an 8×8-inch pan—the bake time will be similar.

For the Oatmeal Base & Crumble Topping

The base and topping use simple pantry ingredients:

  • white sugar
  • brown sugar
  • flour
  • cinnamon
  • old-fashioned oats
  • melted butter (I prefer salted)
  • 1 large egg

Whisk the sugars, flour, cinnamon and oats together, then stir in the melted butter and egg. The mixture should resemble damp sand; it’s fine if a little flour remains visible. When squeezed in your palm it should hold together. Reserve about 1 1/2 cups of the mixture for the topping and press the remainder firmly into the bottom of the lined pan.

Step by step collage photos of how to make berry crumble bars. For the Juicy Berry Middle

The filling is simply mixed berries tossed with lemon juice, sugar and cornstarch. Use fresh or frozen berries—any combo of chopped strawberries, raspberries, blueberries and blackberries works well. The lemon brightens the flavor (you won’t taste it distinctly), and the cornstarch thickens the juices so the bars hold together.

Tips for the berry filling:

  • If using frozen berries, do not thaw before mixing.
  • Chop strawberries and any large blackberries; leave raspberries and blueberries whole.
  • If using frozen berries, reduce the lemon juice to 1 tablespoon instead of 2.
  • Always include the cornstarch to prevent an overly runny filling.
  • If the berries release a lot of juice when mixed, don’t pour all of that excess into the pan—discard some liquid to avoid watery bars.

Spoon the berry mixture over the pressed oatmeal base, then crumble the reserved oat mixture over the berries, forming larger clumps by squeezing the topping between your hands. Bake for about 45–50 minutes, until the top is golden and you can see the berry filling bubbling through.

Cooling is essential: let the pan sit at room temperature for about 30 minutes, then refrigerate until completely cold—this can take up to 4 hours. Cut the bars only once fully chilled so they slice cleanly and don’t turn into a jammy mess. Allow at least 4 hours total cooling before slicing. Overhead view of 3 mixed berry oatmeal crumble bars stacked on top of each other.

These crumble bars are great served cold from the fridge or warm with a scoop of ice cream. They work equally well for dessert or breakfast and make a dependable, simple fruit dessert.

Ingredients (for a 9×13-inch pan, about 15 bars):

  • Oatmeal Base & Topping: 2 1/2 cups all-purpose flour, 1 teaspoon cinnamon, 1 1/2 cups old-fashioned oats, 1 cup white sugar, 1/2 cup brown sugar, 1 cup melted butter, 1 large egg.
  • Berry Layer: 4 1/2 cups mixed berries (strawberries, raspberries, blueberries, blackberries), 1/2 cup white sugar, 2 tablespoons lemon juice (use 1 tablespoon with frozen berries), 3 tablespoons cornstarch.

Method summary:

  • Preheat oven to 350°F. Line a 9×13 pan with parchment or greased foil.
  • Mix flour, cinnamon, oats and sugars. Stir in melted butter and egg until the mixture is like wet sand.
  • Reserve 1 1/2 cups of the oat mixture for topping. Press the rest into the bottom of the prepared pan.
  • Toss berries with sugar, lemon juice and cornstarch until evenly coated. Spoon over the base, spreading gently.
  • Scatter the reserved crumble on top, forming larger clumps by squeezing in your hands.
  • Bake 45–50 minutes, until golden and bubbling. Cool 30 minutes at room temp, then refrigerate until completely cold (up to 4 hours). Slice into bars and store airtight in the fridge for up to 4 days.
3 berry crumble bars made with oatmeal and brown sugar stacked on a white plate.

Mixed Berry Crumble Bars

Buttery oatmeal crumbles and sweet, juicy berries make these Mixed Berry Crumble Bars impossible to resist. The bars use the same oat mixture for the base and crumble topping with a summertime berry filling.
Prep: 15
Cook: 50
Total: 1 5
Servings: 15 bars

Ingredients

Oatmeal Base & Topping

  • 2 1/2 cups all-purpose flour
  • 1 teaspoon cinnamon
  • 1 1/2 cups old-fashioned oats*
  • 1 cup white sugar
  • 1/2 cup brown sugar
  • 1 cup butter, melted
  • 1 large egg

Berry Layer

  • 4 1/2 cups mixed berries**
  • 1/2 cup white sugar
  • 2 tablespoons lemon juice, freshly squeezed (use 1 tbsp if berries are frozen)
  • 3 tablespoons cornstarch

Instructions 

  • Preheat the oven to 350°F (175°C).
  • Line a 9×13-inch pan with parchment, leaving an overhang, or line with foil and lightly grease.
  • In a large bowl, stir together the flour, cinnamon, oats and sugars.
  • Stir in the melted butter and egg until the mixture looks like wet sand.
  • Reserve about 1 1/2 cups of the mixture for the topping. Press the remaining mixture into the bottom of the prepared pan.
  • In a large bowl, toss the berries with the sugar, lemon juice and cornstarch until evenly coated.
  • Spoon the berry mixture over the base, spreading gently.
  • Crumble the reserved oatmeal mixture over the berries, squeezing to form larger clumps.
  • Bake 45–50 minutes, until the top is golden and the filling is bubbling.
  • Cool on a wire rack for 30 minutes, then refrigerate until completely chilled (up to 4 hours). Slice into bars once cold. Store in an airtight container in the fridge for up to 4 days.

Notes

*Do not use instant or quick oats—use old-fashioned or large-flake oats for best texture.

**A mix of strawberries, raspberries, blueberries and blackberries works well. Slice strawberries and large blackberries; leave raspberries and blueberries whole. If using frozen berries, do not thaw.

***If using frozen berries, reduce lemon juice to 1 tablespoon. Lemon juice can be substituted with water, but lemon improves the flavor.