01 - Preheat the oven to 350°F. Lightly butter a 2-quart baking dish and set aside.
02 - In a large mixing bowl, combine the blueberries, sliced peaches, granulated sugar, cornstarch, lemon juice, and vanilla extract. Toss gently until the fruit is evenly coated, then spread the mixture in a single 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 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 remaining.
04 - Scatter 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 deeply 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 accompanied by vanilla ice cream or freshly whipped cream.