Mix the sourdough with the water, add the flours, salt. Knead for 4 minutes in first and for 4 minutes in 2nd (7 on a stand mixer). Let the dough rise in a warm and humid area (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 (240°C) with a cast iron pot.
Once the oven is warm, remove the baking dish and transfer the dough ball to it. Bake for 30 minutes with the lid on, then for 25 minutes without the lid at 430°F (225°C).
