Stir together flour, salt, and yeast in a large bowl. Add 1 cup water and buttermilk. Using your hand as a claw, stir together until no dry bits of flour remain and all clumps are broken up. Do not knead. Cover bowl tightly with plastic wrap; let stand at room temperature until it’s very bubbly and the consistency resembles pancake batter, at least 12 hours or up to 18 hours.

Turn out dough on a well-floured work surface. (It will be very loose.) Incorporating as little flour from surface into dough as possible, shape it into a taut ball by folding edges in toward middle and repeating until it begins to develop structure and hold its shape, 12 to 16 times. Turn dough seam side down on a clean (not floured) work surface. Heavily sprinkle top with flour. Using a dough scraper, pull the dough across the work surface, rotating it as you drag it, developing tension and forming a taut ball. Coat generously in flour.

Line an 8-inch proofing basket or bowl with a linen or cotton towel (not terry cloth). Dust the towel generously with flour; sprinkle with cornmeal. Using a dough scraper, lift and carefully invert dough ball into prepared bowl so the smooth, rounded side faces down and the seam faces up. Cover bowl with a kitchen towel. Let stand at room temperature until doubled in size, 1 to 2 hours. Preheat oven to 500°F with rack in lower third position. Meanwhile, place a cast-iron Dutch oven (5 quart or larger) with lid in oven to preheat about 20 minutes.

Remove hot Dutch oven from oven. Carefully invert dough ball into Dutch oven. Working quickly using a thin, sharp knife, make 3 (about 4-inch-long, 1/4-inch-deep) cuts across the top of the loaf. Cover bread with Dutch oven lid; return to oven. Reduce oven temperature to 450°F. Bake, covered, until loaf has increased in volume and turned a shiny, pale light brown, about 25 minutes. Remove lid. Continue baking until loaf is golden brown and a thermometer inserted in center registers 205°F, 12 to 18 minutes. Transfer to a wire rack; cool 1 hour.