Step 1: In a large mixing bowl, combine the active sourdough starter with the warm water. Stir gently to mostly dissolve the starter. Add the bread flour and mix with your hands or a Danish dough whisk until just combined and no dry flour remains. The dough will be shaggy. Cover the bowl with a damp cloth or plastic wrap and let it rest for 30 minutes to 1 hour (autolyse).
Step 2: After the autolyse, sprinkle the salt evenly over the dough. Using damp hands, gently work the salt into the dough by squeezing and folding it for about 2-3 minutes until well distributed.
Step 3: Cover the bowl again and let the dough rest. Over the next 3-4 hours, perform 4-5 sets of “stretch and folds” every 30-45 minutes. With damp hands, grab a portion of the dough from one side, stretch it upwards, and fold it over the center. Rotate the bowl and repeat 3-4 more times. After the last set, let the dough complete its bulk fermentation, covered, at room temperature (70-75°F / 21-24°C) until it has increased in volume by about 30-50% (this can take 6-12 hours total, depending on your environment and starter).
Step 4: Lightly flour your work surface. Gently turn the dough out onto the surface. Using a bench scraper, gently shape the dough into a loose round or rectangle. Avoid deflating it too much. Let it rest, uncovered, for 20-30 minutes (pre-shaping).
Step 5: Lightly flour a banneton (proofing basket) or a bowl lined with a floured cloth. Gently shape the dough into a taut round or oval, creating tension on the surface. Place the dough, seam-side up, into the prepared banneton. Cover it with plastic wrap or place it in a large plastic bag. Refrigerate for 12-16 hours (cold proof).
Step 6: About 45 minutes to 1 hour before you plan to bake, place your Dutch oven (with lid) into your oven and preheat to 450°F (232°C).
Step 7: Carefully remove the hot Dutch oven from the oven. Gently invert the cold dough from the banneton directly into the hot Dutch oven. Use a sharp razor blade or lame to score the top of the dough. Immediately place the lid back on the Dutch oven. Bake for 30 minutes with the lid on. Remove the lid and continue baking for another 10-15 minutes, or until the crust is a deep golden brown and the internal temperature reaches 200-210°F (93-99°C).
Step 8: Carefully remove the bread from the Dutch oven and place it on a wire rack to cool completely for at least 2 hours before slicing. Slicing too early can result in a gummy interior.