In a bowl, mix the apple slices, white sugar, 2 1/2 tablespoons of flour, and 1 teaspoon of cinnamon together until thoroughly combined; spread out into a 9x13-inch baking dish. In a separate large bowl, mix the melted butter with nutmeg and 2 teaspoons of cinnamon; stir in the brown sugar, oats, 1 cup of flour, and mini marshmallows until thoroughly combined. Spread the marshmallow topping evenly over the apples, and gently flatten the topping to a smooth layer.
Bake in the preheated oven until the apples are tender and the topping is golden brown, 30 to 35 minutes.
