In a bowl, combine the apple slices, melted butter, brown sugar and cinnamon. Toss all of the ingredients together until they are equally coated. Pour the filling into your pan. Set aside.
In a small bowl, combine the flour, brown sugar, cold butter and cinnamon. Using a pastry cutter or your hands, cut the butter into the other ingredients until they are all combined. The mixture will be sandy. Sprinkle on top of the apple filling.
Bake for 30-40 minutes or until the apple filling is bubbly and the apples are soft and the streusel is golden brown. If you notice the streusel is browning too quickly, place a piece of tin foil on the top.
Once finished, let cool for 5 minutes. Serve warm, top with ice cream and enjoy!