Mix the levain with all other ingredients, except the salt. Make sure that all the flour is hydrated.
Let it rest for an hour.
Add salt and stretch and fold the dough 30 times and let it rest for an hour in a new well-oiled kitchen bowl.
Repeat two times. You can find detailed instructions about stretching and folding in the recipe notes.
Divide the dough in two and form each piece into loaves. Let the dough rest for a few minutes before you place it seam-side-up in a floured towel lined kitchen bowl or banneton. You can read more about how to form loaves in the recipe notes. Place them in plastic bags.
Allow the loaves to rise until doubled in size. The time depends on the surrounding temperature.
Preheat your oven to 500ºF / 260ºC with two oven plates. One to bake the bread on and one just below.
Place one loaf on the upper plate with the seam side down. Score it and pour some water on the plate below.
Turn down the temperature immediately to 430ºF / 220ºC and bake for 40 minutes.
Take out the loaf from the oven and let it cool on a wire rack. Use an oven mitt.
Turn up the temperature and repeat the process with the other loaf.