Preheat the oven to 350°F (175°C).

In a mixing bowl, combine blueberries, sliced peaches, granulated sugar, lemon juice, and cornstarch. Toss gently until the fruit is coated.

Transfer the fruit mixture to a greased baking dish.

In another bowl, mix together flour, rolled oats, brown sugar, melted butter, and cinnamon until crumbly.

Sprinkle the crumble topping evenly over the fruit mixture in the baking dish.

Bake in the preheated oven for 30 minutes or until the topping is golden brown and the fruit is bubbling.

Let cool for a few minutes before serving.