Preheat your oven to 350°F (175°C).
Prepare the apples by peeling, coring, and slicing them into thin wedges.
In a large mixing bowl, combine the sliced apples with the granulated sugar, lemon juice, cinnamon, nutmeg, and flour.
Transfer the apple mixture into a greased baking dish.
In another bowl, mix together the rolled oats, brown sugar, melted butter, and salt until combined.
Spread the oat mixture evenly over the apple filling.
Bake in the preheated oven for 35-40 minutes or until the topping is golden brown and the apples are bubbling.
Remove from the oven and let cool for a few minutes before serving.