Preheat your oven to 350 degrees.

In a large bowl, gently mix the sliced apples with the sugar, flour, and cinnamon.

Place the apple mixture into an 8x8 inch glass baking dish or similar casserole dish.

Melt the butter in the microwave, using a large measuring cup or small bowl.

Add the oats, brown sugar and flour and mix until well combined.

Distribute the topping mixture over the apples.

Bake at 350 degrees for 35 minutes, or until the apples are bubbling and the topping is toasted and golden.

Serve with vanilla ice cream.