Make apple mixture and topping ahead of time and store in the refrigerator in resealable bags until ready to use.
Combine brown sugar, oatmeal, flour, cinnamon and nutmeg. Cut in butter until mixture resembles rice.
Peel, core and slice apples. Mix in lemon juice, cinnamon, cornstarch and salt until apples are completely coated.
Place prepared apples in the bottom of a 12 inch cast iron stock pot. Sprinkle topping mixture evenly over apples. Place lid on pot and place on rack over open campfire. Cover top of lid with hot coals. Cook for approximately 20-30 minutes, checking often. Cooking times will vary.