How to Make It
Stir together warm water and sourdough starter in a large bowl until evenly incorporated and mostly dissolved. Add bread flour; stir well until no dry bits of flour remain. Cover bowl with a kitchen towel; let stand at warm room temperature until almost doubled in volume, in volume and small bubbles appear on surface, 2 to 3 hours.
Add 1 1/4 cups of the warm water to levain in bowl; stir until evenly incorporated and mostly dissolved. Add bread flour; using your hands, mix together until no dry bits of flour remain. Cover bowl with a kitchen towel; let rest at warm room temperature 30 minutes.
Uncover bowl, and sprinkle dough with salt; add remaining 1/4 cup warm water. Using a grabbing/squeezing motion with your hands, incorporate salt and water into dough until mostly smooth, 2 to 4 minutes. (Dough will initially separate into clumps but eventually form a homogenous mixture.) Cover bowl with a kitchen towel; let rest at warm room temperature until dough increases slightly in volume and becomes strong and elastic, 3 to 4 hours, stretching and folding dough back over itself in bowl every 30 minutes.
Lightly dust top of dough with bread flour. Turn dough out, floured side down, onto a clean work surface. Using a bench scraper or your palms, pull the dough towards you across work surface, rotating about 90 degrees as you drag it, to develop tension and form a taut ball. Drag and rotate the dough ball 4 to 5 times. Place dough ball on a work surface lightly dusted with bread flour. Cover with a kitchen towel; let rest 30 minutes.
Line a proofing basket (about 8 1/2 inches wide) or medium bowl (about 9 inches wide) with a clean linen kitchen towel or large cloth napkin, and generously dust with wheat germ or white rice flour; set aside. Lightly dust top of dough with bread flour. Using a bench scraper or your palms, pull dough towards you across work surface, rotating about 90 degrees as you drag it, to develop tension and re-form a taut ball. Drag and rotate dough ball 4 to 5 times. Let dough ball rest on work surface to help seal bottom seam for 1 minute.
Using a bench scraper or your hands, lift the dough ball, and carefully invert into prepared proofing basket so that the smooth, rounded side is facing down and the seam side is facing up. Cover basket with a kitchen towel; let stand at room temperature until dough slightly increases in volume, about 30 minutes. Immediately proceed to Step 7, or chill dough in covered basket at least 8 hours or up to 18 hours to develop the flavor even more.
If using a gas oven, place rack in lower third position; if using an electric oven with the heating element in the bottom, place a pizza stone on the lowest oven rack to help concentrate the heat (if you don’t have a pizza stone, just skip it) and place a second rack in the lower third position for the bread itself. Preheat oven to 500°F for 1 hour and let dough continue proofing at room temperature while oven preheats. If dough was chilled overnight, remove from refrigerator, and let stand at room temperature while oven preheats. During final 20 minutes of preheating, place either a 10-inch or larger round bread pan such as a cloche with lid, or a 5-quart or larger cast-iron Dutch oven with lid (enameled or non-enameled) in oven to preheat.
If the surface of dough is sticky, lightly dust with wheat germ or white rice flour. Once bread pan is fully preheated, invert dough ball onto an 11-inch square sheet of parchment paper so that the rounded side is now on top. Using a bread lame or a very sharp, thin knife, score a 6-inch-long, 1/3-inch-deep slash down center of loaf.
Remove bread pan from oven. Remove lid from bread pan, and carefully place parchment paper with dough ball inside preheated bread pan. Place lid back on bread pan, and return to oven. Reduce oven temperature to 450°F. Bake, covered, until the boule has increased in volume, opened up at the score, and turned a shiny pale-light brown, about 25 minutes. Remove lid and continue baking boule until golden brown and an instant-read thermometer inserted in center of bread registers 200°F, 8 to 14 minutes more. Transfer boule from bread pan to a wire rack to cool; remove and discard parchment paper. Let cool completely, about 2 hours. Slice and serve.