Preheat the oven to 350 degrees F (175 degrees C).

Combine flour, oatmeal, cinnamon, nutmeg, and brown sugar in a large bowl; cut in butter until mixture is crumbly.

Spoon half of the mixture into the bottom of a 9x13-inch baking dish; pat down with the back of a spoon to form an even layer.

Cover crumb mixture with apple slices, then sprinkle apple slices with remaining crumb mixture.

Bake in the preheated oven until apples are tender, about 45 to 50 minutes.