Mix flour, oat flakes, brown sugar and salt. Add butter and knead with the dough hook of the hand mixer. Grease the fat pan of the oven (approx. 32 x 39 cm) and press approx. 2/3 of the dough onto it to an even base.
Bake in the preheated oven, mid-height (electric oven: 200 °C/ convection oven: 175 °C/ gas: see manufacturer) for approx. 15 minutes until brown.
Peel and quarter apples and cut out the core. Cut apples into cubes and sprinkle with lemon juice. Mix cinnamon and 3 tbsp. sugar, mix into the apples. Mix quark, 100 g sugar, vanillin sugar, lemon peel and eggs.
Stir in the starch last.
Spread the quark mixture on the pre-baked base and spread the cinnamon apples on top. Crumble the rest of the dough as crumbles on top. In the preheated oven, middle shelf (electric cooker: 200 °C/ convection oven: 175 °C/ gas: see
manufacturer) bake for about 40 minutes. Let the cake cool down on the fat pan. Cut the cake into pieces. Add caramel sauce.