01 - Preheat the oven to 350°F (175°C). 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 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 incorporate it using your fingertips or a pastry blender until the mixture resembles coarse crumbs with some pea-sized butter pieces remaining.
04 - Scatter the crumble topping evenly over the fruit filling, ensuring full coverage across the surface.
05 - Bake on the center rack for 35 to 40 minutes, until the topping is deeply golden brown and the fruit filling is bubbling around the edges.
06 - Allow the crumble to rest for 10 to 15 minutes before serving. Serve warm, optionally accompanied by vanilla ice cream or freshly whipped cream.