Feed your starter. Feed your sourdough starter 4–8 hours before baking so it becomes active, bubbly, and near its peak. It should double in size and smell pleasantly tangy.
Mix dough. In a large bowl, stir together the water and sourdough starter, then add the flour and mix until incorporated. Cover and rest for 30 minutes, then add the salt and work it in with a spoon or your hands. Cover and rest for 30 minutes.
Strengthen the dough with stretch-and-folds/kneading. Over the next 2 hours, perform a set of stretch-and-folds every 30 minutes (3–4 sets total). The dough should become smoother and more elastic. Grab the dough on one edge and stretch it up, then fold it over the opposite edge, repeat on every edge. If stretch and folds intimidate you, just knead the dough a bit every 30 minutes, then place it back in the bowl, cover, and rest.
Bulk ferment until puffy. After the last stretch and fold, cover and let the dough rise at room temperature until it looks airy, shows bubbles along the sides, and has increased in size by about 50%. If your kitchen is cooler than 70F, consider placing the dough in the oven with the light on or in a warm place.
Pre-shape and rest. Turn the dough onto a lightly floured surface, gently shape into a loose round, and rest 20–30 minutes.
Final shape. Shape into a tight round by pulling the dough toward you to create surface tension. Place seam-side up in a well-floured banneton basket or towel-lined bowl.
Cold proof. Cover and refrigerate 8–24 hours to improve flavor, structure, and scoring before baking.
Preheat oven and Dutch oven. Place your Dutch oven or pot with a lid inside the oven and preheat to 475°F for at least 30–45 minutes.
Turn out and score. Remove dough from the refrigerator, flip onto parchment paper, and score the top with a sharp knife or razor.
Bake covered, then uncovered. Lower the temperature of the over to 425F, then carefully lower the dough into the hot Dutch oven. Cover and bake 30 minutes, then uncover and bake 10-15 minutes more until deep golden brown.
Cool completely. Transfer to a rack and cool at least 1 hour or overnight before slicing so the crumb sets and avoids gumminess.