Melt butter in saucepan. Remove from heat when melted. Add grated apple (including skins) then add sugar and egg and beat until well mixed. Stir in sifted dry ingredients until just mixed. Turn mixture into greased cake tin.
To make topping: Melt butter, remove from heat, stir in rolled oats, sugar and cinnamon.
Spread topping evenly over cake.
Bake cake at 180°C for 30-40 mins or until sides shrink from tin.