Preheat oven to 375 degrees F (190 degrees C). Lightly grease an 8-inch square baking dish.
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 resembles coarse cornmeal; sprinkle over apples.
Bake in preheated oven until topping is golden, 40 to 45 minutes. Cool slightly before serving.
