3 cups bread flour, plus more for work surface and sprinkling
2 1/2 teaspoons kosher salt
1/4 teaspoon active dry yeast
1 cup water
1/2 cup whole buttermilk, or 1/2 cup milk + 1.5 tsp vinegar or lemon juice
Fine yellow cornmeal, for sprinkling
Stir together flour, salt, and yeast in a large bowl. Add 1 cup water and buttermilk. If you’re using regular milk instead of buttermilk, mix the milk and vinegar or lemon juice 5 minutes before mixing it into the rest of the ingredients.
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. Carefully invert the dough ball into the Dutch oven.
Quickly make 3 cuts across the top of the loaf, about 4-inch-long & 1/4-inch-deep. Cover the bread with the Dutch oven lid & return it to the oven.
Reduce the oven temperature to 450°F. Bake, covered, until the loaf has increased in volume and turned a shiny, pale light brown, about 25 minutes.
Remove the lid. Continue baking until the loaf is golden brown, and a thermometer inserted in the center registers 205°F ~12 to 18 minutes.
Transfer to a wire rack; cool 1 hour.