Peel and core 250 g apples and cut into wedges. Mix the butter with the egg yolk, sugar, milk, oat flakes, cornstarch and baking powder well. Fold in the stiffly beaten egg white and apple pieces and pour everything into a greased dish.
Peel the last apple, cut out the core and cut the apple into rings, cover the casserole with it. Bake in the preheated oven for approx. 35 minutes at 200 ° C or gas mark 3-4.