01 - Preheat your oven to 350°F. Lightly grease an 8×8-inch baking dish with butter or nonstick spray.
02 - In a large mixing bowl, combine the mixed berries, granulated sugar, cornstarch, vanilla extract, and lemon juice. Toss gently until the berries are evenly coated, then transfer the mixture to the prepared baking dish, spreading it into an even layer.
03 - In a separate bowl, whisk together the all-purpose flour, rolled oats, light brown sugar, sliced almonds, and salt. Add the cubed cold butter and vanilla extract. Using your fingertips, rub the butter into the dry ingredients until the mixture resembles coarse, pea-sized crumbs.
04 - Scatter the crumble topping evenly over the berry filling, ensuring full coverage across the surface of the dish.
05 - Place the dish in the center of the oven and bake for 35 minutes, or until the topping is deeply golden and the berry juices are bubbling around the edges.
06 - Remove from the oven and let the crumble rest for 10 minutes before serving. Serve warm, optionally with a scoop of vanilla ice cream or a dollop of freshly whipped cream.