Mix both types of flour well in a bowl. Make a hollow on the left and right. Pour the salt into one well and the yeast into the other. Both ingredients shouldn`t touch each other at first. Then pour the warm water into the bowl and finally add the sourdough.
Knead the whole thing into a dough, if possible by hand (duration: about 3 - 5 minutes). Then lightly dust the dough with flour and cover with a damp cloth, leave to rise for approx. 30 minutes at approx. 20 ° C (room temperature).
On a well-floured work surface, knead the dough with well-floured hands and shape (grind) a loaf of bread from it. It is important to create a certain surface tension on the lower side.
Place the loaf with the bottom side up on a baking sheet lined with baking paper, dust with flour, cover and let rest for another hour.
Before baking the bread, make several cross-sections so that the bread does not crack (too large) on the sides. Bake the bread at approx. 180 ° C top / bottom heat for 50 minutes.