Make the Preferment (Day 1): In a large bowl, stir together the flour, yeast (or sourdough culture), and cold water until fully combined and no dry flour remains. Cover the bowl and let it rest at room temperature overnight for 12 to 16 hours.
Prepare the Dough (Day 2): Add the warm water to the preferment and break it apart using your fingers. Mix until mostly dissolved. Stir in the flour, salt, and yeast, mixing by hand until all dry bits disappear.
Bulk Fermentation: Let the dough rise at room temperature for 2 hours. During the first hour, fold the dough every 20 minutes using a bowl fold technique (lifting and folding the dough onto itself). After the final fold, let it rest undisturbed for the second hour.
Strengthen the Dough: Cover and rest the dough for 20 minutes. Then perform 8 to 12 bowl folds. Let it rest again for 20 minutes, and follow with 2 to 3 coil folds using wet hands. Once the dough becomes smooth and elastic, shape it into a ball and let it sit for 1 hour.
Divide and Preshape: Lightly flour a surface and use a scraper to remove the dough. Cut it in half and gently form each piece into a ball. Let rest seam side down, covered, for 15 minutes.
Final Shaping and Proofing: Lightly flour two 9 x 3-inch proofing baskets. Shape each dough round into a boule or bâtard, place them seam side up into baskets, and cover. Let rise for 45 minutes to 1 hour, until soft and puffy.
To Bake on a Stone or Steel: Preheat oven to 475°F with a baking stone and empty cast-iron skillet. Flip the dough onto parchment paper, score the tops, and transfer them onto the hot stone. Pour 1 cup of hot water into the skillet to steam. Bake 32 to 38 minutes until crust is firm and golden brown.
To Bake in a Covered Baker: Preheat oven and baker to 475°F for 1 hour. Transfer one dough round into the hot baker using parchment paper as a sling. Score the top, cover, and bake for 20 minutes. Remove lid and bake uncovered for another 12 to 18 minutes until golden brown. Repeat with second loaf.
Cool: Transfer finished loaves to a wire rack and let cool fully before slicing.