Mix 250 g flour, 1 teaspoon sugar and 1⁄2 teaspoon salt in a bowl. Warm the milk lukewarm. Crumble yeast into it and dissolve it while stirring. Add milk and yeast mixture and 25 g butter to the flour mixture.
Knead with the dough hooks of the mixer to a smooth dough. Cover the dough and let it rise in a warm place for about 40 minutes.
In the meantime peel and halve the onions and slice or cut them into fine slices. Finely dice the bacon, fry until crispy in a large pan and remove. Add oil and heat. Add the onions and fry them for about 15 minutes, turning them over, without letting them brown.
Take it off the stove. Mix with the bacon.
Grease a springform pan (26 cm Ø) and dust with flour. Knead the yeast dough well on a floured work surface with floured hands and roll out round (approx. 32 cm Ø). Put it into the springform pan and press the rim 2-3 cm high.
Cover and leave to rise in a warm place for about 30 minutes.
Preheat oven (electric cooker: 175 °C/circulating air: 150 °C/gas: see manufacturer). Mix sour cream and eggs. Season with salt, pepper and nutmeg. Sprinkle the dough base with breadcrumbs. Spread the onion mixture on top.
Pour egg icing over it. Bake in a hot oven on the lowest shelf for 50-60 minutes. Remove from oven and let rest for about 10 minutes. Remove from the springform pan and cut into pieces.