In a large mixing bowl, stir together the warm water and sourdough starter until mostly dissolved. Add the bread flour and salt. Mix with a dough whisk, fork, or your hand until fully combined and no dry patches remain. The dough will look shaggy and feel sticky.
Cover the bowl and let the dough rest at room temperature for 45 minutes.
After resting, knead the dough in the bowl for about 5 minutes. Use your hand and a flexible dough scraper to fold the dough over itself. The scraper is helpful for lifting the dough and cleaning your hands. This short kneading session helps kickstart gluten development.
Cover again and let the dough rest. Complete 3 sets of stretch and folds, spaced 30–45 minutes apart. For each session, perform 4 to 6 folds: wet your hand, lift and stretch the dough from one side, and fold it over to the other side. Rotate the bowl and repeat on all sides. This strengthens the dough and improves structure.
After the final fold, cover and allow the dough to bulk ferment at room temperature. This step typically takes 4 to 8 hours, depending on your kitchen temperature. Look for the dough to rise noticeably, become domed and jiggly, and form visible bubbles along the sides and bottom of the bowl (a clear bowl is helpful here).
Once bulk fermentation is complete, dust a clean work surface lightly with flour. Gently turn out the dough. Stretch it into a loose rectangle (it doesn’t need to be large), then fold each side inward toward the center like folding a letter. Starting from one short end, roll the dough up into a log.
To shape the dough into a boule, use the push-and-pull method: gently cup your hands around the dough and pull it toward you along the counter to create tension across the surface. Then rotate it slightly and repeat, pushing it away, then pulling it back toward you again. Continue turning and repeating this motion until the dough forms a tight, round ball with a smooth surface.
Place the dough into a banneton dusted with rice flour or lightly floured bread flour. If you don’t have a banneton, use a medium bowl lined with a thin, floured tea towel. Cover and transfer to the refrigerator to cold proof for 8 to 12 hours. (Note you can cold proof up to 48 hours. If cold proofing more than 48 hours, this can cause your loaf to be flat when baked.)
When ready to bake, preheat your oven to 475°F with a Dutch oven inside for at least 30 minutes.
Remove the dough from the fridge, turn it out onto parchment paper, and score the top with a sharp razor or bread lame.
Carefully transfer the dough (on parchment) into the hot Dutch oven. For extra steam, you can drop 2 to 3 ice cubes between the parchment and the wall of the Dutch oven just before closing the lid. The added steam can help promote better oven spring and a shinier, crisper crust.
To ensure the bread is fully baked, use an instant-read thermometer inserted into the center of the loaf. It should register between 205–210°F when done.
Let the bread cool on a wire rack for at least an hour before slicing.