Line a suitable, fireproof pan with baking paper, stir the whole eggs with the sifted powdered sugar, vanilla sugar, cinnamon and lemon zest until very frothy, then carefully fold in the biscuit crumbs and preheat the oven to 180 degrees. Wash the apples, peel, cut in half and remove the core, then cut into thin wedges and drizzle with lemon juice, line the bottom and edge of the baking dish with the apple wedges, pour in the egg mixture, in the preheated oven
Bake for about 40 minutes, then turn the casserole out of the mold and carefully peel off the baking paper, sprinkle with cinnamon sugar before serving.