Step 1

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.

Step 2

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.

Step 3

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.

Step 4

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, ¼-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.