Preheat the oven to 350°F (175°C).

Prepare the apples by peeling, coring, and slicing them.

In a large bowl, toss the sliced apples with sugar, lemon juice, and cinnamon.

Transfer the apple mixture into a greased baking dish.

In another bowl, mix together flour, oats, baking powder, and salt.

Stir in the melted butter until the mixture is crumbly.

Sprinkle the topping mixture 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.

Let it cool slightly before serving.