In a large bowl, stir together the starter and water to form a slurry. Add the flour and stir to combine; the leaven will be somewhat stiff. Cover with plastic and allow to ferment at room temperature until puffy and aromatic, 8 to 12 hours.

Combine the water and flours in a medium bowl and mix until the flour is hydrated and no lumps remain. Cover with plastic and allow to autolyze at room temperature for 1 to 2 or up to 4 hours. Sprinkle the salt over the dough. Break the sourdough leaven into small pieces and add it to the dough. Mix with your hands until the leaven is completely incorporated and no visible streaks remain. Cover with plastic and allow to bulk ferment at room temperature for about 3 to 4 hours, stretching and folding in the bowl every 30 to 45 minutes.

When the dough is puffy and increased in volume by about one-third, preshape it into a round boule. Cover with plastic and allow to bench rest for 10 to 30 minutes. Final shape according to your preference and banneton shape and place the dough seam side up in the well-floured banneton. Cover with a towel and then plastic and refrigerate for 8 to 24 hours.

Remove your loaf from the refrigerator and allow it to come to room temperature for about 1 hour or until it passes the poke test. Preheat a Dutch oven (preferably enamel coated) to 500°F (260°C) on the middle rack of the oven for 20 minutes. Sprinkle a touch of cornmeal onto a piece of parchment paper cut to fit the bottom of the Dutch oven and carefully flip your loaf onto it, seam-side down. Score the top of the loaf about ¼ to ½ inch deep with a razor blade or lame. Carefully lower the parchment paper and loaf into the preheated Dutch oven, position the lid, and return it to the oven. Reduce the oven temperature to 480°F (249°C) and bake with the lid on for 20 minutes. Remove the lid and bake for another 12 to 20 minutes or until the crust is brown.

Note from Sarah: I prefer my bottom crust to be as thick and dark as possible, but if you are using a non-enameled cast-iron pot, you will need to tip the loaf out of the Dutch oven after about 25 minutes of baking time to prevent the bottom from burning before the loaf is done. This must be done carefully to avoid burning yourself. Finish baking directly on the middle rack for an additional 12 to 20 minutes.

Recipes and photography excerpted from Toast & Jam