Peel and wash the potatoes and boil them in salted water. Put 375 g flour in a bowl, press a depression in the middle. Warm milk, crumble yeast, dissolve in it. Pour into the hollow, mix with some of the flour to make a pre-dough. Cover and leave to rise in a warm place for about 20 minutes
Drain the potatoes, collecting 100 ml of water. Mash potatoes, mix with water, cool down a little. Add to pre-dough, knead. Leave to rise for about 20 minutes.
Clean the leek, wash it. Peel onions. Cut both into rings. Fry in hot oil. Grease a fat pan (approx. 35 x 40 cm). Whisk sour cream, eggs and spices
Knead the dough, dust with flour. Roll out on the fat pan, press the rim up. Spread onion mixture and glaze on top. Bake in the preheated oven (electric oven: 200 °C/circulating air: 175 °C/gas: level 3) for 40-45 minutes