01 - Preheat the oven to 350°F. Grease a 9-inch square or round baking dish with butter or nonstick spray.
02 - In a large mixing bowl, toss the blueberries and sliced peaches with granulated sugar, cornstarch, lemon juice, and vanilla extract until the fruit is evenly coated. Spread the mixture in an even 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 cut it in using your fingers or a pastry cutter 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 - Place the dish in the center of the oven and bake for 35 to 40 minutes, until the topping is deeply golden brown and the fruit juices are bubbling around the edges.
06 - Remove from the oven and let the crumble rest for 10 to 15 minutes before serving. Serve warm on its own or accompanied by vanilla ice cream or freshly whipped cream.