For the crumble dough, mix flour, oat flakes, sugar, 1 pinch of salt and butter in pieces with the dough hooks of the mixer to a crumbly crumble dough. 1⁄3 Put the crumble aside. Grease a fat pan (approx. 32 x 39 cm).
Spread the rest of the crumble on top. Press to a base with your hands. Bake in the preheated oven (electric cooker: 200 °C/circulating air: 175 °C/gas: see manufacturer) for approx. 15 minutes until the base is lightly browned.
For the topping peel, wash, quarter and core apples. Cut into small pieces and mix immediately with lemon juice. Mix 3 tbsp. sugar and cinnamon. Mix into the apples.
Mix quark, 100 g sugar, vanillin sugar, lemon peel and eggs. Stir in starch. Spread the quark mixture on the pre-baked base. Spread the cinnamon apples on top. Sprinkle with the rest of the crumble dough.
Bake in the preheated oven (electric cooker: 200 °C/circulating air: 175 °C/gas: see manufacturer) for approx. 30 minutes. Remove the cake and let it cool down. Cut into pieces. Serve with caramel sauce, for example.