01 - Preheat the oven to 350°F. Lightly butter a 9-inch baking dish and set aside.
02 - In a large bowl, combine the mixed berries, granulated sugar, cornstarch, lemon juice, and vanilla extract. Toss gently until the berries are evenly coated. Transfer the mixture to the prepared baking dish, spreading it into an even layer.
03 - In a separate bowl, whisk together the flour, oats, sliced almonds, brown sugar, cinnamon, and salt. Add the cold cubed butter and vanilla extract. Using your fingers or a pastry cutter, blend the butter into the dry ingredients until the mixture resembles coarse crumbs with some pea-sized pieces remaining.
04 - Scatter the crumble topping evenly over the berry filling, covering the surface completely.
05 - Bake for 35 minutes, or until the topping is deep golden brown and the berry filling is bubbling around the edges.
06 - Allow the crumble to cool for at least 10 minutes before serving. Serve warm with vanilla ice cream or whipped cream if desired.