Grease a fat pan (deep baking tray; approx. 32 x 39 cm). Preheat the oven (electric cooker: 200 °C/circulating air: 175 °C/gas: see manufacturer). For the crumble dough, mix flour, oat flakes, brown sugar and 1 pinch of salt.
Add 360 g butter in pieces. Knead into crumbles with the dough hooks of the mixer. 2⁄3 Press the dough on the tray to form a base. Pre-bake in a hot oven for about 15 minutes until tenderly golden brown.
For the filling, wash the lemon hot, dab dry. Finely grate the peel. Squeeze lemon. Peel, quarter and core apples. Cut into cubes and sprinkle with lemon juice. Mix cinnamon and 3 tbsp. sugar, mix with apples.
Mix quark, 100 g sugar, vanilla sugar, lemon peel and eggs. Stir in starch.
Spread the quark mixture on the pre-baked crumble base. Spread the cinnamon apples on top. Crumble the rest of the dough as crumbles. Bake in a hot oven at the same temperature for about 35 minutes. Let the cake cool down on the baking tray.
Cut into about 20 pieces. Sprinkle with caramel sauce to serve.