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 together the blueberries, sliced peaches, granulated sugar, cornstarch, lemon juice, and vanilla extract until the fruit is evenly coated. Spread the mixture uniformly into the prepared baking dish.
03 - In a separate bowl, combine the flour, rolled oats, brown sugar, cinnamon, and salt. Add the cold cubed butter and cut it in using your fingertips or a pastry cutter 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 - Bake for 35 to 40 minutes, or until the topping is golden brown and the fruit juices are bubbling around the edges.
06 - Allow the crumble to cool for 10 to 15 minutes before serving. Serve warm with vanilla ice cream or freshly whipped cream if desired.