01 - Preheat oven to 350°F. Lightly grease an 8-inch square baking dish with butter or nonstick spray.
02 - In a large bowl, toss the mixed berries with granulated sugar, cornstarch, and vanilla extract until evenly coated. Spread the berry mixture in a single, even layer across the bottom of the prepared baking dish.
03 - In a separate bowl, whisk together the rolled oats, all-purpose flour, sliced almonds, brown sugar, cinnamon, and salt. Pour in the melted butter and vanilla extract, stirring until the mixture forms coarse, evenly moistened crumbs.
04 - Sprinkle the crumb topping evenly over the berry filling, covering the surface as uniformly as possible.
05 - Bake on the center rack for 35 minutes, or until the topping is deeply golden and the berry juices are actively bubbling around the edges.
06 - Allow the bake to rest for 10 minutes at room temperature before serving. Serve warm, optionally accompanied by a scoop of vanilla ice cream or a dollop of freshly whipped cream.