01 - Preheat the oven to 350°F. Lightly grease a 9-inch baking dish with butter or nonstick spray.
02 - In a large bowl, combine the mixed berries, granulated sugar, cornstarch, vanilla extract, and lemon juice. Toss gently until the berries are evenly coated, then spread the mixture uniformly across the bottom of the prepared baking dish.
03 - In a separate bowl, whisk together the flour, rolled oats, sliced almonds, brown sugar, and salt. Add the cold cubed butter and vanilla extract. Using your fingertips or a pastry cutter, work the butter into the dry ingredients until the mixture resembles coarse crumbs with a few larger clumps throughout.
04 - Sprinkle the crumble 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 deep golden brown and the berry filling is bubbling around the edges.
06 - Remove from the oven and let the crumble rest for at least 10 minutes before serving. Serve warm, accompanied by vanilla ice cream or freshly whipped cream if desired.