PREHEAT THE OVEN AND PREPARE THE PAN

1. Preheat the oven to 350°F (175°C).
2. Grease an 8×8-inch metal baking pan and line with parchment paper, leaving an overhang on all sides for easy removal. Set aside.

PREPARE THE CRUMBLE TOPPING AND BASE

1. In a large bowl, whisk together the rolled oats, gluten-free 1:1 flour, almond flour, brown sugar, granulated sugar, baking powder, and salt, breaking up any large clumps of brown sugar.
2. In a separate bowl, stir together the melted butter, vanilla extract, and almond extract.
3. Pour the wet ingredients over the dry ingredients and mix until evenly combined and fully coated.
4. Remove 3/4 cup (lightly packed) of the mixture and refrigerate (this will be the topping).
5. Press the remaining mixture firmly into the bottom of the prepared pan and smooth into an even layer.

PREPARE THE BERRY FILLING

1. In a medium saucepan, combine the blueberries, blackberries, raspberries, lemon zest, lemon juice, sugar, and cornstarch.
2. Cook over medium heat, stirring frequently, until the mixture begins to bubble.
3. Reduce the heat and continue cooking, stirring constantly, until thickened and jammy, about 3–5 minutes. The mixture should coat the back of a spoon.
4. Remove from heat and stir in the vanilla extract.
5. Let cool for 5 minutes before assembling.

ASSEMBLE THE BARS

1. Pour the berry filling over the crust and spread into an even layer.
2. Sprinkle the chilled crumble topping evenly over the berries.
3. Finish with the remaining 1 tablespoon of rolled oats.

BAKE THE BARS

1. Bake for 28–32 minutes, or until the crumble topping is lightly golden and the filling is bubbling.
2. Let the bars cool completely in the pan (about 1 1/2 to 2 hours).
3. Lift out using the parchment, slice into bars, and serve.

Store the bars covered at room temperature for up to 2 days, or refrigerate for up to 5 days.