Melt 100 g butter and let it cool down. Warm the milk lukewarm. Sift 300 g flour and rye flour into a bowl, make a small depression and crumble yeast into it. Mix with the warm milk and a little flour from the rim to make a pre-dough.
Cover and leave to rise in a warm place for about 30 minutes.
Add lukewarm butter and 1 teaspoon salt and knead everything with the dough hooks of the mixer to a smooth dough. Cover the dough and let it rise in a warm place for another 1 hour until the volume has doubled.
In the meantime dice bacon finely. Peel and roughly dice the onions. Melt 2 tablespoons of butter in a large pan. Fry the bacon and onions until the onions are glassy but not browned. Leave to cool.
Mix onion-bacon mix with sour cream, 100 g flour and eggs. Season with salt and 1 tbsp. caraway seeds.
Preheat oven (electric cooker: 200 °C/circulating air: 175 °C/gas: see manufacturer). Grease a baking tray (approx. 35 x 40 cm). Roll out the yeast dough to the size of the baking tray and place on top. Spread the onion mix on the dough.
Bake in a hot oven for 40-50 minutes. Take out, cut into pieces and arrange.