Sprinkle apples with lemon juice. In separate bowl, combine oats, flour, cinnamon, salt and butter. Spray a metal bowl that will fit loosely into pressure cooker with non-stick spray. Place alternate layers of apples and oat mixture into bowl, beginning and ending with apples. Cover bowl firmly with aluminum foil and place on rack in cooker. Add the 2 cups of water. Secure cover, and make sure pressure gauge is in place.