Preheat your oven to 350°F and grease or spray an 8-inch or 9-inch casserole or baking dish.
Place the blueberries (2 cups), peaches (2 cups), lemon juice (1 teaspoon), melted butter (2 tablespoons), brown sugar (1/2 cup), cornstarch (1 tablespoon), and cinnamon (1 teaspoon)in a large mixing bowl. Stir together to combine & coat the fruit evenly.
Pour the fruit filling into your prepared baking dish. Spread evenly and set aside.
To make the crisp topping, place the oats (1 1/2 cups), flour (3/4 cup), sugar (1/2 cup), cinnamon (1/2 teaspoon), and salt (1/4 teaspoon) in a bowl. Stir to combine.
Pour the melted butter (1 stick) over the ingredients in the bowl and stir until thoroughly incorporated
Spoon the topping over the blueberry peach filling and gently spread until the filling is covered evenly.
Bake for 45-55 minutes or until the crisp topping is lightly browned and the filling is bubbly and warm.
Serve warm, as is, or top with whipped cream or vanilla ice cream and enjoy.