Make the sourdough the day before by mixing all the ingredients and letting it rest overnight, covered.
The next day, knead 300 g of sourdough with the remaining ingredients until a homogeneous dough is formed. Let the dough rise briefly, then work according to Stretch and Fold and shape into a loaf. Put the loaf in a proofing basket and cover and let rest for 2 - 3 hours.
Preheat the oven to 230 degrees.
Bake the dough for 15 minutes and then again for 25 - 35 minutes at 200 degrees. The bread is ready when it passes the tapping or core temperature test.