01 - Preheat oven to 350°F. Lightly butter a 2-quart baking dish and set aside.
02 - In a large bowl, toss together the blueberries, sliced peaches, granulated sugar, cornstarch, lemon juice, and vanilla extract until the fruit is evenly coated. Spread the mixture in a uniform layer across the bottom of the prepared baking dish.
03 - In a separate bowl, combine the flour, rolled oats, brown sugar, cinnamon, and salt. Add the cold cubed butter and rub it in with your fingertips or a pastry blender until the mixture resembles coarse crumbs with some pea-sized pieces of butter remaining.
04 - Sprinkle the crumble topping evenly over the fruit filling, covering the surface as uniformly as possible.
05 - Bake for 35 to 40 minutes, or until the topping is deep golden brown and the fruit filling is bubbling around the edges.
06 - Allow the crumble to cool for 10 to 15 minutes before serving. Serve warm with a scoop of vanilla ice cream or a dollop of whipped cream if desired.