Onions peel and chop finely. Wash the apple thoroughly, quarter and core it. Cut the fruit flesh into small cubes as well. Sauté onions in 2 tablespoons of hot oil until translucent. Add the apple pieces and fry briefly.
Set mixture aside. Stir together the sour cream and egg yolk. Season with salt, pepper and marjoram. Grate cheese very finely. For the dough knead flour, quark, baking powder, milk and 6 tablespoons of oil. Season with a little salt.
Divide the dough into approx. 16 pieces of the same size, form into balls and roll out on a floured work surface until round (approx. 10 cm Ø). Place 6 or 4 pieces on a baking tray lined with baking paper, spread each with a tablespoon of sour cream, spread the onion mixture on top.
Sprinkle with cheese and bake one after the other in a preheated oven (electric oven: 200 °C/ gas: level 3) for about 25 minutes.