For the yeast dough mix flour and dry yeast in a bowl. Add 30 g fat, milk, egg, salt and sugar and knead with the dough hook of the hand mixer to a smooth dough.
Cover and leave in a warm place for about 30 minutes. In the meantime, clean and wash the leek and cut it into fine rings. Dice the bacon and drain in a pan. Add leek and sauté briefly.
Stir in sour cream, season with pepper. Let it cool down. Roll out the yeast dough on a floured work surface to a rectangle (30x35 cm). Spread leek and bacon mixture on top. Cut lengthwise into 5 strips of 6 cm width.
Roll up and place in a springform pan (18 cm Ø) lined with baking paper. Allow to rise again for another 15 minutes and bake in a preheated oven (electric: 200 °C/ gas: level 3) for 25-30 minutes.
Meanwhile, for the sauce, peel and finely dice the onion. Fry in the remaining hot fat. Pour in wine and let it boil down a little. Pour on cream and stir in cheese. Bring to the boil and stir in the sauce thickener.
Season to taste with salt, pepper and nutmeg. Melt the rest of the butter and spread it on the still hot cake. Arrange the cake on a plate and add the sauce.