Melt 50 g butter and let it cool down. Warm 100 ml milk and 50 ml water lukewarm. Dissolve yeast in it. Mix 250 g flour and 1 teaspoon salt, press a depression in the middle. Pour in yeast milk, pour liquid butter to the rim.
Knead everything with the dough hooks of the hand mixer to a smooth dough. Cover and leave to rise in a warm place for 30-40 minutes (the firm dough should increase by about 1/3). Grease a round spring plate (30 cm Ø) and dust with flour.
Knead the yeast dough again. Roll out round (approx. 36 cm Ø) on a little flour. Fold the round dough sheet once, place it in the form, fold it apart again and press it up at the edge by about 3 cm.
Cover and leave to rise for 15-20 minutes.
Peel and finely chop the onion. Grate cheese. Preheat oven (electric cooker: 200°C/circulating air: 175°C/gas: level 3). 50 g flour and.
Stir 400 ml milk in a pot until smooth, bring to the boil while stirring continuously. Add onion.
Simmer everything at low heat while stirring for about 5 minutes. Stir in cheese and melt. Remove the pot from the stove. Whisk eggs and sour cream, stir in. Season with salt, pepper and nutmeg.
Pour the cheese mixture onto the dough base. Bake in a hot oven on the lowest shelf for 35-40 minutes. If the cheese mixture becomes too brown, cover with aluminium foil for the last 10 minutes of the baking time.