Wash, peel and quarter the apples and cut out the core. Cut apple quarters into slices and sprinkle with lemon juice. Mix quark, milk, oil, 75 g sugar, lemon peel and salt.
Mix 300 g flour and baking powder and knead with the curd mixture. Then continue kneading with your hands to a smooth dough. Roll out the dough on a greased baking tray (approx. 34 x 38 cm) and cover with apples.
Sprinkle deep-frozen blueberries into the spaces in between. Melt butter for the crumbles. Put the rest of the flour, sugar and vanilla sugar in a mixing bowl and add the hot butter. Use the dough hooks of the hand mixer to knead into crumbles.
Sprinkle crumbles over the fruit and bake in the preheated oven (electric: 200 °C / gas: level 3) for approx. 30 minutes. Let it cool down and cut it into about 24 pieces.