1. Preheat oven to 350 degrees Fahrenheit and spray a 9×13-inch Pyrex dish or disposable pan with cooking spray. Combine sliced apples, flour and cinnamon in a bowl and then place in prepared pan.
2. In a second bowl, combine flour, margarine, sugar, graham cracker crumbs, oats, and walnuts (if using) with your fingers. Crumble on top of apples and sprinkle with cinnamon.
3. Bake uncovered in preheated oven for one hour.