Stew the apples in a little water (add a tablespoon of caster sugar if you like your stewed fruit sweeter).
Mix oats, brown sugar, flour and cinnamon together in a bowl.
Rub butter into oat mixture to form crumble.
Place apple into a baking dish and pop the crumble on top.
Bake in a 170 degree oven for 30 minutes (or until the crumble has browned slightly).
Serve with ice-cream.