Preheat your oven to 400°F (200°C).

In a large bowl, combine the sliced apples, mixed berries, granulated sugar, cornstarch, cinnamon, and nutmeg. Toss to coat evenly. Stir in the melted butter.

Pour the fruit mixture into a 9 or 10-inch cast-iron skillet, spreading it evenly.

In a separate medium bowl, whisk together the flour, rolled oats, brown sugar, and salt for the crumble topping.

Add the cold butter cubes to the dry ingredients. Use a pastry blender, fork, or your fingertips to cut the butter into the mixture until it resembles coarse crumbs.

Sprinkle the crumble topping evenly over the fruit mixture in the skillet.

Place the skillet on a baking sheet (to catch any drips) and bake for 25-30 minutes, or until the topping is golden brown and the fruit is bubbly.

Let the skillet cool for at least 10-15 minutes before serving. This allows the filling to thicken.