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, combine the blueberries, sliced peaches, granulated sugar, cornstarch, lemon juice, and vanilla extract. Toss gently until all the fruit is evenly coated. Transfer the mixture to the prepared baking dish and spread it into an even layer.
03 - In a separate bowl, whisk together the all-purpose flour, rolled oats, packed brown sugar, ground cinnamon, and salt. Add the cold cubed butter and use your fingers or a pastry cutter to cut it into the dry ingredients 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 completely.
05 - Place the dish in the center of the oven and bake for 35 to 40 minutes, until the topping is deep 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.