Sift the flour into a bowl and make a depression in the middle. Crumble the yeast, dissolve it with sugar in 150 ml lukewarm water. Pour into the hollow. Mix with some flour from the edge. Cover and leave to rise in a warm place for 15-20 minutes
In the meantime peel and finely dice the onions. Dice the bacon as well and fry it in a pot or pan without fat. Add onions and fry for 2-3 minutes while turning until transparent. Season with a little salt and pepper and let cool off
Add oil and 1/2 teaspoon salt to the pre-dough on the flour rim. Knead everything to a smooth dough. Cover the dough and let it rise for another 20-30 minutes. Grease baking tray or fat pan (32 x 39 cm), sprinkle with flour. Knead the dough and roll it out. Press well into the corners
Mix quark, crème fraîche and egg yolk. Season with some salt, pepper and nutmeg. Spread the icing on the dough. Sprinkle the onion-bacon mixture over it. Bake in the preheated oven (electric cooker: 250 °C/ convection oven: 225 °C / gas: level 5) for 12-15 minutes