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 bowl, toss the blueberries, sliced peaches, granulated sugar, cornstarch, lemon juice, and vanilla extract together until the fruit is evenly coated. Spread the mixture uniformly into the prepared baking dish.
03 - In a separate bowl, whisk together the all-purpose flour, rolled oats, packed brown sugar, cinnamon, and salt. Add the cold cubed butter and use your fingers or a pastry cutter to cut it in until the mixture resembles coarse crumbs with some pea-sized pieces 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 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 on its own or accompanied by vanilla ice cream or freshly whipped cream.