Directions:

1. To prepare levain: Combine starter, 1/2 cup water, 1/3 cup plus 1 tablespoon all-purpose flour and 1/3 cup plus 1 tablespoon whole-wheat flour in a medium bowl. Cover loosely and let stand in a warm spot until bubbly and doubled in size, 4 to 6 hours.
2. To prepare dough: Combine 3 1/3 cups plus 1 tablespoon all-purpose flour and 2 1/4 cups whole-wheat flour in the bowl of a stand mixer fitted with a dough hook. Set to the lowest speed and slowly add 2 3/4 cups plus 2 tablespoons warm water. Mix on low speed until the flour is moist, about 20 to 30 seconds, being careful not to overmix. Cover the mixing bowl and let stand for 20 minutes to allow the water to completely hydrate the flour before mixing the final dough.
3. Remove the mixing bowl cover and turn the mixer on low. Add salt, then spoon in the levain (from Step 1) and mix until combined, about 2 minutes.
4. Transfer the dough to a 10- to-12-quart plastic food-storage container. Reach under one end of the dough, pull up a handful until there is resistance, then stretch it back over the top of the dough mass and drop. Turn the container a quarter turn and repeat the fold. Repeat two more times for a total of 4 folds. Cover and let stand for 30 minutes. Repeat the folding procedure 4 more times, every 20 minutes, to build the gluten and strengthen the dough. Cover and let stand until doubled in size, 6 to 12 hours.
5. Liberally flour the inside of two medium bowls or proofing baskets.
6. Turn the dough out onto a floured surface, sprinkle flour on top and split it in half. With floured hands, roll each half into a ball. Place the dough balls into the prepared bowls or baskets. Cover with a clean kitchen towel and let rise until the dough is puffy, 2 to 4 hours. To test, press gently with a floured finger. If the depression springs back to the original shape in a few seconds, allow the dough to proof longer. If it doesn't spring back, it is ready to bake.
7. Meanwhile, place a large cast-iron pot with lid in the oven and preheat to 475 degrees F. After reaching 475 degrees F, wait at least 20 minutes for the pot to heat up before baking.
8. Carefully remove the pot from the oven and remove the lid. With floured hands, pick up the dough and carefully place the loaf inside the pot. (It is OK to gently drop it in from a height of 3 to 4 inches to avoid burning yourself.) Replace the lid and transfer to the oven. (Place the second loaf in the refrigerator while the first one bakes.) Bake for 30 minutes, remove the lid and continue baking until browned, about 15 minutes.
9. Remove the loaf from the oven and carefully transfer the bread to a wire rack. Let cool for at least 30 minutes before cutting.
10. Repeat Steps 7-9 for the second loaf.