Preheat oven to 350°F. Spray a 9-inch pie plate (an 8 or 9-inch square baking dish will work fine too) with nonstick cooking spray and set aside.
To make the topping:
In a mixing bowl, whisk together the flour, brown sugar, cinnamon, and salt until well combined. Add the cold cubed butter and using a pastry cutter or fork, cut the butter into the dry ingredients until the mixture starts to come together and is crumbly. Stir in the oats and transfer to the refrigerator while you make the filling.
To make the filling:
Add the sliced apples, sugar, flour, cinnamon, and nutmeg to a large mixing bowl and stir until all of the apple slices are well coated. Scoop the apples into the prepared baking dish. Remove the topping from the refrigerator and sprinkle on top of the filling.
Bake at 350°F for 40-50 minutes or until the topping is lightly golden brown and the juices are bubbling around the edges.
Remove from the oven and transfer to a wire rack to cool slightly before serving.