Preheat oven to 350 degrees.
Arrange apple slices evenly in prepared baking dish. Sift flour, sugar, cinnamon, and salt in a bowl.
Cut in butter using a pastry blender or 2 knives until mixture becomes fine crumbs. Sprinkle crumb topping over apples.
Bake at 350 degrees for 35-40 minutes, depending on how dark you like the crumbs. Cool slightly before serving.