For the buttermilk loaf, dissolve the yeast in lukewarm water (be careful not to overheat!). Mix the dry ingredients. Add water, buttermilk and sourdough and mix.
Knead with a food processor for about 10 minutes on the lowest setting (can also be done by hand, of course). Cover and let rise for 1/2 hour. Work (shape) the bread.
Cover and let rise in a cooking basket (or in a bowl lined with a cloth) for another 1/2 hour. Heat oven to 230 °C with a fireproof vessel standing on the bottom.
After the rising time, turn the bread out onto a baking tray and cut into it. Pour plenty of water into the container in the oven – WARNING: DANGER OF BURNING!
Put the bread in the middle rack and bake for about 50-55 minutes, turning the temperature down to 185 °C. After about 4-5 minutes, let the steam escape from the oven.
After the baking time, the bread should sound hollow at the bottom when tapped, otherwise bake a few minutes longer. Let cool on a rack before cutting.