01 - Preheat oven to 350°F. Grease a 9-inch square or round baking dish with butter or nonstick spray.
02 - In a large bowl, toss 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 a pastry cutter or your fingers until the mixture resembles coarse crumbs with some pea-sized butter pieces remaining.
04 - Sprinkle the crumble topping evenly over the fruit filling, covering the surface completely.
05 - Bake for 35 to 40 minutes 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.