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 uniformly into the prepared baking dish.
03 - In a separate bowl, whisk together 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 butter pieces remaining.
04 - Evenly sprinkle the crumble topping 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, optionally topped with vanilla ice cream or freshly whipped cream.