01 - Preheat oven to 350°F. Lightly grease an 8-inch baking dish.
02 - In a large mixing bowl, toss the mixed berries with granulated sugar, cornstarch, and vanilla extract until evenly coated. Transfer the mixture into the prepared baking dish, spreading it into an even layer.
03 - In a separate bowl, combine the flour, rolled oats, brown sugar, sliced almonds, cinnamon, and salt. Add the cold cubed butter and rub it into the dry ingredients using your fingertips until the mixture resembles coarse crumbs with some pea-sized pieces remaining.
04 - Evenly scatter the crumble topping over the berry filling, covering the surface as uniformly as possible.
05 - Bake for 35 minutes, or until the topping is deep golden brown and the berry juices are bubbling around the edges.
06 - Allow the crumble to rest for 10 minutes before serving. Serve warm on its own or accompanied by vanilla ice cream or freshly whipped cream.