Peel the banana and mash it with a fork. Mix with the sour cream, egg, juice and zest of lemon, vanilla sugar and cinnamon until smooth.
Peel, halve and core the apples. Cut into narrow wedges, layer in a baking dish, sprinkle with the brown sugar, drizzle with rum and pour the banana cream over them.
Sprinkle the casserole with flaked almonds and bake for about 20 minutes.