Mix the sourdough with the water, add the flours, salt. Knead 4 minutes in first and 4 minutes in 2nd (7 on a stand mixer). Let the dough rise in a warm and humid place (the oven with a bowl of hot water is an ideal place) for 4 hours. Shape the dough into a ball. Heat the oven to 475 ° F with a cast iron pot.
When the oven is hot, take out the casserole and put the ball of dough. Cook for 30 minutes with the lid, then 25 minutes without the lid at 430 ° F.
