Pre-heat oven to 350 degrees.
Prep apples by washing, peeling, and slicing them.
Heat cast iron over medium low heat. In a small bowl, mix sugar, cinnamon, nutmeg, and salt.
Add to butter and stir. Add apples and coat with butter and cook for 5 minutes until apples are softened.
In a medium bowl, add oats, cinnamon, flour, sugar, cinnamon, and salt and mix well. Add cubed cool butter and cut in with a pastry cutter or pulse with a food processor.
Sprinkle on top of apples in cast iron.
Place in center rack in oven and bake for 40-45 minutes.
Serve with ice cream or caramel sauce!