01 - Preheat oven to 350°F. Grease a 9-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. Spread the mixture in an even layer across the bottom of the prepared baking dish.
03 - In a separate bowl, whisk together the flour, rolled oats, brown sugar, cinnamon, and salt. Add the cubed cold butter and vanilla extract. Using your fingertips or a pastry cutter, rub the butter into the dry ingredients until the mixture resembles coarse crumbs with some pea-sized pieces remaining. Fold in the sliced almonds.
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 juices are bubbling around the edges.
06 - Allow the crumble to rest for 10 minutes before serving. Serve warm with a scoop of vanilla ice cream or a dollop of whipped cream if desired.