Melt the butter and let it cool down. Mix 100 ml milk and 50 ml water, heat up and dissolve the yeast in it. Mix 250 g flour and about 1 teaspoon of salt in a bowl, add yeast milk and butter. Knead with the dough hook of the hand mixer for 2-3 minutes. Cover and leave to rise in a warm place for about 30 minutes.
Knead the yeast dough again and roll out on a floured work surface to a circle of 33-34 cm Ø. Line a greased baking tray (approx. 30 cm Ø) with the dough and press the edge lightly. Prick the base several times with a fork, cover and leave to rise in a warm place for another 15-20 minutes. In the meantime peel and finely dice the onion. Mix the remaining milk and flour in a pot and bring to the boil while stirring. Add onions and simmer at low heat for 4-5 minutes. Mix eggs and sour cream. Pull the pot off the stove. Add cheese and sour cream mixture and mix everything smooth. Season with salt, pepper and nutmeg.
Mix eggs and sour cream. Pull the pot off the stove. Add cheese and sour cream mixture and mix everything smooth. Season with salt, pepper and nutmeg. Pour the cheese mixture onto the pastry base and bake in the preheated oven (electric range: 200 °C/ gas: level 3) for 35-40 minutes. Best served hot or lukewarm. Makes approx. 12 pieces