Preheat your oven to 375°F.
Grease a 9x9-inch baking dish with butter or cooking spray. (A round dish that is 2.5 to 3 quarts will also work.)
In a large bowl, combine the fruit with the sugar and lemon juice. All the mixture to sit while you prepare the topping.
4-5 cups apples, 2 cups fresh or frozen blackberries, ¼ cup granulated sugar, 1 tablespoon lemon juice
In another bowl, mix together the oats, flour, brown sugar, cinnamon, and salt. Using the back of a fork, or a pastry cutter, add the butter to work it into the dry ingredients until the mixture resembles coarse crumbs. Set aside.
1 cup old-fashioned oats, ½ cup all-purpose flour, ½ cup light brown sugar, ½ teaspoon ground cinnamon, ¼ teaspoon salt, 8 tablespoons unsalted butter
Toss the fruit with the remaining filling ingredients until evenly coated. Transfer the mixture to the prepared baking dish.
2 tablespoons all-purpose flour, 1 teaspoon ground cinnamon, ½ teaspoon ground nutmeg, 1 tablespoon cornstarch
Sprinkle the crumble topping evenly over the fruit mixture in the baking dish.
Bake in the preheated oven for 35 to 40 minutes, or until the filling is bubbly, and the topping is golden brown.
Remove from the oven and let it cool slightly before serving.