Make sure your sourdough culture is active. If your sourdough has been in the refrigerator, take it out 2 to 3 days before you plan to bake. Feed it daily to make sure it's strong and very active before you make the bread.

Make the leaven and let it sit overnight. The night before you plan to make the dough, place all the leaven ingredients in a large bowl and mix thoroughly to form a thick batter. Cover and let stand at room temperature overnight, about 12 hours.

Test that the leaven is ready. Generally, if the surface of the leaven is very bubbly, it's ready to be used. To double check, drop a small spoonful of the leaven in a cup of water; if the leaven floats, it's ready.

Dissolve the salt. Place 50 grams (about ¼ cup) of the water and the salt for the dough in a small bowl. Set aside, stirring occasionally to make sure the salt dissolves.

Mix the leaven and water. Add the remaining 475 grams (2 cups) of water for the dough to the bowl of leaven. Stir with a spatula or use your hands to break up and dissolve the leaven into the water.

Add the flour. Add the flour and stir with a rubber spatula until there are no more bits of dry flour and it forms a very shaggy dough.

Rest the dough (30 minutes, or up to 4 hours). Cover the bowl with plastic wrap or a clean kitchen towel. Let the dough rest for at least 30 minutes or up to 4 hours.

Mix in the salt. Pour the dissolved salt over the dough. Work the liquid and salt into the dough by pinching and squeezing the dough.

Begin folding the dough (2 ½ hours). To fold the dough, grab the dough at one side, lift it up, and fold it over on top of itself. Let the dough rest 30 minutes, then repeat.

Let the dough rise undisturbed (30 to 60 minutes). Once you've finished the folds, cover and let the dough rise undisturbed for 30 to 60 minutes.

Divide the dough. Sprinkle some flour on a work surface and turn the dough out on top. Use a pastry scraper to divide the dough in half.

Shape the dough into loose rounds. Use your pastry scraper to shape each one into loose rounds.

Rest the dough (20 to 30 minutes). Once both pieces of dough are shaped, let them rest for 20 to 30 minutes.

Prepare 2 bread proofing baskets, colanders, or mixing bowls. Line with clean kitchen towels and dust them heavily with flour.

Shape the loaves. Dust the top of one of the balls of dough with flour, flip it over, and shape the loaf.

Transfer to the proofing baskets. Dust the tops and sides of the shaped loaves generously with flour and place them into the proofing baskets upside down.

Let the dough rise (3 to 4 hours, or overnight in the fridge). Cover the baskets loosely and let them rise.

Heat the oven to 500°F. Place two Dutch ovens or other heavy-bottomed pots with lids in the oven.

Transfer the loaves to the Dutch ovens. Carefully remove one heated Dutch oven from the oven and tip the loaf into the pot.

Score the top of the loaf. Use a lame or sharp knife to quickly score the surface of the loaves.

Bake the loaves for 20 minutes. Cover and bake.

Reduce the oven temperature to 450°F and bake another 10 minutes.

Remove the lids and continue baking 15 to 25 minutes.

Bake another 15 to 25 minutes. Continue baking uncovered until the crust is deeply browned.

Cool the loaves completely. Transfer them to wire racks to cool completely.