Preheat oven to 375°F (190°C) and lightly grease an 8×8-inch or 9-inch round baking dish.

1. In a large bowl, toss sliced apples and blueberries with lemon juice, granulated sugar, brown sugar, cinnamon, and cornstarch until evenly coated.
2. Pour the fruit mixture into the prepared baking dish and spread into an even layer.
3. In a separate bowl, combine flour, oats (if using), brown sugar, and salt. Add cold butter and rub with fingers or a pastry cutter until mixture forms coarse crumbs.
4. Evenly sprinkle the crumble topping over the fruit.
5. Bake for 40–45 minutes, or until the topping is golden brown and the filling is bubbling around the edges.
6. Let cool for at least 10 minutes before serving. Serve warm with ice cream or whipped cream if desired.