1. Preheat the oven to 400 degrees and grease an 8 x 8- inch baking dish.
2. Peel and core the apples and cut them into thin slices. Spread the slices in the prepared baking dish.
3. Combine the flour, sugar, cinnamon, if using, in a medium bowl. Cut in the butter with a pastry blender or fork until the mixture is crumbly. Spread evenly over the apples.
4. Bake uncovered until golden brown, 40 to 45 minutes.
To double this recipe, bake it in a 13 x 9-inch pan for 50 to 60 minutes.