01 - Preheat 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 cut it in using a pastry cutter or your fingertips 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 preheated oven and bake for 35 to 40 minutes, or until the topping is deeply golden brown and the fruit juices are bubbling around the edges.
06 - Remove from the oven and let cool for 10 to 15 minutes before serving. Serve warm on its own or accompanied by a scoop of vanilla ice cream or a dollop of freshly whipped cream.