Mix the flour and buckwheat flour. Make a well in the middle and crumble the yeast into it (alternatively use 2 bags of dry yeast). Add 300 ml of lukewarm water in portions and mix the yeast (mixture) with flour and the newly added water each time. Cover the mixture with flour and let it rise (pre-dough).
Peel the potatoes, cut them into small pieces and add 300 ml of water and 2 teaspoons of salt to the top. As soon as the potatoes are tender, use the potato masher to mash them in the remaining liquid. Then whip the resulting mashed potatoes with the whisk. If this becomes too firm, add more water so that at the end a creamy, not too firm mass is created. Set the mashed potatoes on one side to cool.
As soon as the pre-dough has risen (wider cracks in the flour surface) and the mashed potatoes have cooled down (lukewarm), add the mashed potatoes to the bowl with the pre-dough. Work the starter dough, mashed potatoes and the rest of the flour together (wooden spoon or mixer). If the dough is too dry, add more water. Knead the dough by hand on the work surface into a smooth dough that pulls longer threads. If the dough sticks to the work surface, add more flour.
Place a baking dish with approx. 200 ml of hot water on the bottom of the oven. Preheat the oven to 250 ° C (top / bottom heat).
Shape the risen dough into a round loaf. To do this, roughly shape the dough into a round dumpling and then work the dumpling with floured hands so that the edge is always pushed down. This results in a smooth, taut surface, which then results in a nice, evenly shaped, smooth bread.
Place the shaped dough on a baking tray lined with baking paper. Brush the dough generously with water. Let rise as long as the oven needs to heat up (approx. 20 min.).
Tip: If possible, do not wet the transition from the dough to the baking paper with water, as this will cause the bread to stick to the baking paper and tear. However, this is only an optical damage, it does not affect the taste.
Put the bread on the tray on the middle sliding bar (if available half a bar lower) in the oven. Bake for 15 minutes at a very high temperature (250 ° C or above), then reduce the temperature to 200 ° C. Bake the bread at 200 ° C for another 45 minutes (total baking time approx. 60 minutes).