01 - Preheat the oven to 350°F. Lightly grease a 9-inch square baking dish with butter or cooking spray.
02 - Combine the mixed berries, granulated sugar, cornstarch, vanilla extract, and lemon zest in a large bowl. Toss gently until berries are evenly coated.
03 - Spread the berry mixture evenly across the bottom of the prepared baking dish.
04 - In a separate bowl, whisk together the almond flour, oats, sliced almonds, brown sugar, cinnamon, and salt until well combined.
05 - Pour the melted butter and vanilla extract into the dry mixture. Stir until evenly moistened and mixture forms small clumps.
06 - Sprinkle the almond crumble mixture evenly over the berry layer, covering completely.
07 - Bake for 35 minutes until the topping is golden brown and the berry filling is bubbling around the edges.
08 - Let the bake cool for 10–15 minutes before serving. Serve warm with vanilla ice cream or Greek yogurt if desired.