1 1/4 cup (10 ounces) water
1 1/2 teaspoons yeast
2 cups (16 ounces) ripe sourdough starter (See Recipe Notes)
4 to 4 1/2 cups (18-20 1/4 ounces) all-purpose flour
1 tablespoon kosher salt (or 1 scant tablespoon table salt)
Combine the water and the yeast in the bowl of a stand mixer or in a large mixing bowl. Give the yeast a few minutes to dissolve completely. Stir in the sourdough starter until the starter is mostly dissolved (a few stringy bits are ok).
Add 4 cups of the flour and the salt, and stir to make a shaggy dough. With the dough hook attachment and your mixer on low speed, knead the dough for about 8 minutes. Alternatively, turn the dough out on a lightly floured counter and knead by hand. Add flour 1 tablespoon at a time as needed if the dough becomes sticky like bubble gum, but try to avoid adding too much. The dough is finished kneading when it comes together into a smooth ball that's slightly tacky to the touch and holds a ball-shape in your hand. (See more info: How to Tell When Dough is Kneaded.)
Clean out the mixing bowl, film it with a little oil, and return the dough to the bowl. Turn it a few times to coat with oil, then cover. Let the dough rise at warm room temperature until doubled in bulk, 1 1/2 to 2 hours.
Once risen, turn the dough out onto a lightly floured counter and divide it in two. Shape each half into rough balls and let them rest for 20 minutes. Meanwhile, grease two 8 1/2 x 4 1/2 loaf pans.
Shape each half into a sandwich loaf. (See more info: How to Shape a Sandwich Loaf.) Transfer the loaves to the loaf pans and cover loosely. Let the loaves rise until they're starting to puff over the rim of the pan, 1 to 1 1/2 hours. Alternatively, put your loaves in the refrigerator and let them rise slowly overnight.
When you see the loaves just starting to reach the rim of the loaf pans, begin preheating the oven to 450°F.
Slash the top of the loaves a few time with a serrated knife or baking lame, and slide them immediately into the oven. For a crispier crust, spritz the inside of the oven with water using a water spritzer before closing the oven. Bake for 10 minutes, then reduce the heat to 400°F. Continue baking for another 25 to 30 minutes, until the tops of the loaves are deep golden brown and the loaves sound hollow when tapped on the bottom. (Total baking time is 35 to 40 minutes.)
Shake the loaves out of the loaf pans and let them cool completely on a cooling rack.