In a large metal or glass bowl, whisk together the water and starter until mostly combined. Add the flour and salt. Stir together by hand or with a spatula until the mixture is very shaggy but most of the flour is mixed in.
Cover with a damp kitchen towel or plastic wrap and let it sit at room temperature for 1 hour. (This helps with flavor and kick-starts gluten development.)
Stretch and Fold:
Fill a bowl or measuring cup with several inches of cold water. Uncover your dough.
With the dough bowl directly in front of you, dip your fingers into the water, and then slide them under one edge of the dough down to the bottom of the bowl. (I like to start at the top of the bowl, or 12 o’clock.) Pull that section of dough up, stretch it over the center, and press it down into the dough to help hold it in place. Turn the bowl 90°, or a quarter turn, and repeat 3 more times until you’ve made 4 folds and turns.
Recover the bowl and rest for another 20 minutes.
Repeat the stretch and fold process again. At this point, you can repeat the stretch and fold sequence two more times or you can move on to the rise. (Continuing to stretch and fold will develop a stronger gluten structure resulting in a chewier loaf with a more even crumb structure inside. The flavor and texture will be great either way, so it’s up to your preference and time!)
Bulk Rise:
After the final stretch and fold, shape your dough into a ball. If your bowl is crusted with flour, scrape it clean or transfer your dough to another bowl. Cover and let the dough rise until well puffed, 6 to 12 hours, ideally in a place that is 68°F to 75°F. (See notes on Bulke Rise Time.)
To test if the dough has risen enough, dip your finger in flour and poke the top of the dough. If it mostly springs back, it’s ready! If it holds the indentation without any movement or feels very dense, continue letting it proof and test it every hour. If it looks deflated or very soft, it is likely over-proofed; I’d still continue and bake it! It may bake up flatter but should still taste great.
Shaping the Dough:
Stretch: Dip both hands in cold water. Slip your fingers under the dough on either side and lift the dough up about a foot or two above the counter. It should essentially be draped over your hands. (If the dough is sticking to the bowl, use a dough scraper or a spatula to loosen it.) Place it on the counter.
Pinch: Sprinkle the top of the dough lightly with flour, and flip it over so the floured side is down. Pinch all of the edges into the center, like you are bundling up a sack.
Shape: Flip the dough over so the seams are on the counter. With your hands cupped around the top of the loaf, gently pull the dough ball towards you, tightening the shape. Continue this motion by rotating the dough so that the edges are tightened all around and it forms a nice ball. (You will do this 8-10 times to form a good, tight ball.) Lightly dust the ball with flour.
Line a bowl or colander with a clean smooth kitchen towel and dust generously with flour. With the help of your dough or bench scraper, carefully pick up the dough ball and flip it over, so the top or smooth side is down in the towel. Cover and rise at room temperature for 30 minutes. (You can also place the dough in a large plastic bag and seal it, then refrigerate for up to 36 hours. The cold slows the rise and ages the yeast, resulting in a more sour, deeper flavored loaf.)
After the 30-minute rise, place the loaf in the refrigerator while the oven preheats (at least 1 hour).
Bake:
Place a large cast-iron Dutch oven or large heavy-duty oven-safe pot with the lid on in the oven. Preheat the oven to 500°F for 1 hour. (If you have chilled your loaf for more than 1 hour, remove it from the refrigerator while the oven preheats.)
When ready to bake, remove the loaf from the fridge. Uncover and lightly flour the loaf. Place a piece of parchment paper on top of the bowl and a baking sheet over that. Invert so that the dough falls onto the paper. Remove the bowl and towel.
Using a sharp knife or razor blade, make a shallow cut down the length of the loaf or cut in a decorative design.
Carefully remove the pot from the oven. Using the parchment paper as handles, place the loaf inside the pot and cover with the lid. (If the parchment is hanging out of the pot, just firmly press the lid down to seal it.)
Bake for 20 minutes. Reduce the oven temperature to 450°F.
Carefully remove the lid and continue baking until the crust is a deep golden brown or the internal temperature is 195°F to 205°F, about 20 minutes more. Use the parchment and a spatula to lift the bread out of the pot and place the loaf on a wire rack to cool completely before slicing.
Notes
Active starter: I recommend feeding your starter several hours before you want to start your sourdough loaf. You can use a refrigerated starter that has been fed, then chilled within 3 to 4 days of being fed. Otherwise, feed your starter, then look for it to rise, then start to fall. You want to use it when it's "hungry," aka when it's risen up and starting to deflate again.
Bulk Rise Time: The colder the environment, the longer it will take to rise, and vice versa. In the summer, I leave it on the counter. In the winter, I'll place it in a proofing drawer or in the oven with the light on. You can also choose your environment based on when you want to bake the bread. If you made the dough in the morning and want to bake it in the evening, put it in a warm spot to rise. If you want to slow it down and bake it a day or two later, pop it in the fridge.
Use rice flour for dusting the loaf if you have it. It doesn't burn at the high oven temperature so it produces a really pretty top crust. This is totally optional though!
Dutch oven: If you have a seasoned (non-enamel) cast iron Dutch oven, I recommend using that. If you only have enamel, use an old one or buy one specifically for bread making. Because the pot goes in the oven at a very high temperature for a long period of time, it will likely discolor the inside and/or outside coating. It shouldn't cause damage to enamel that is in good condition, but it will change the appearance.
Let the sourdough bread cool completely before slicing. Slicing it while it is still hot can lead to it feeling gummy and drying out faster.