The night before baking, begin making the levain. Mix the water and yeast together in a small bowl. Add the flour and stir to make a thick batter. Cover with plastic wrap and refrigerate for 8 to 12 hours.

The next day, combine the water and the levain in the bowl of a stand mixer fitted with a paddle attachment at low speed until combined. Add the flours, a little at a time, until a sticky dough forms. Turn off the mixer but do not remove the paddle from the dough. Cover the bowl and allow the mixture to rest for 20 minutes.

At low speed, add the salt until incorporated. Turn off the mixer and remove the paddle. Cover the bowl and allow to rest for 20 minutes.

As the flour absorbs more of the water, over time it will begin to look firmer. With wet hands, pull up one corner of the dough (still in the bowl), stretching it about 10 inches tall and fold it over the top of the dough. Repeat, once quarter of the bowl at a time. Cover the bowl and allow to rest 20 minutes.

Repeat the folding process and 20 minute resting period two more times.

Empty the dough from the bowl onto a well-floured work surface using a dough scraper. The dough will be tacky and ire, do not punch the dough down. Cut the dough in half and place one of the halves in front of you. Cupping your hands a bit, tuck down the sides around the dough, tucking them underneath the now round dough. Repeat with the other half of dough.

Generously flour 2 (8-inch) bannetons, or line 2 (8-inch) colanders with linen and flour them. Turn each dough ball upside down and place it in the banneton. Cover loosely and allow to rest until the dough has risen but has not quite doubled, about 2 hours.

Place your baker's stone in the oven on the lower 3rd rack. Fill a large casserole pan ¾ with water and place it on the bottom rack of the oven. Preheat the oven to 500 degrees Fahrenheit.

If you have a baker's peel, you can dust it with semolina flour and gently turn one loaf out onto the peel, then transfer the loaf from the peel to the baking stone. Or you can turn the loaf out directly from the banneton to the baking stone, but this has to be done quickly so as not to let all the heat out of the oven. Bake for 20 minutes, then lower the oven temperature to 450 degrees Fahrenheit and bake for another 20 minutes.

Remove the loaf and allow it to cool on a wire rack. Repeat this process with the other loaf. The bread can be wrapped in aluminum foil and stored at room temperature for up to 1 day, or in the refrigerator for up to 5 days.