Warm the milk lukewarm. Dissolve yeast in it. Put the flour in a bowl. Press a depression in the middle. Add the milk-yeast mixture, mix with some flour and let it rise covered for 15 minutes.
Meanwhile melt the fat. Add fat and salt to the flour. Knead with the dough hooks of the hand mixer to a smooth dough, cover and leave to rise in a warm place for 30 minutes. Peel onions and cut into rings.
Dice the bacon and leave it out in a pan. Add fat and fry the onions in it. Season with salt, pepper and caraway seeds. Knead the yeast dough again, roll out on a greased baking tray and let it rise for 15 minutes.
Spread the onion-bacon mixture on the dough. Stir sour cream, cheese, eggs and egg yolk until smooth. Pour over the onions and bake in the preheated oven (electric oven: 200 °C/ gas mark 3) for 35 to 40 minutes.