Peel, wash and halve the potatoes and cook in boiling salted water for about 20 minutes. Drain, let it evaporate briefly and mash it finely with a potato masher. Let it cool down lukewarm. Stir sugar beet syrup, sourdough and yeast until smooth. Knead 600 ml lukewarm water, potatoes, 400 g wheat flour, rye flour and 4 teaspoons salt to a smooth dough (dough is very sticky.) Pour dough into a bowl twice as big, close tightly with foil and leave to rise in the fridge overnight. Knead 300 g wheat flour under the soft dough. Halve the dough and form 2 loaves of bread.
Dust fermenting baskets well with flour, add loaves. Cover and leave to rise in a warm place for about 1 hour. Carefully turn out 1 loaf of bread dough onto a baking tray lined with baking paper. Bake in a preheated oven (electric cooker: 225 °C/ convection oven: 200 °C/ gas: level 4) for 35-45 minutes. Let them cool down. Bake the other bread in the same way. Cut the cooled bread into slices, brush with butter and cover with cold cuts
13 hours waiting time