Step 1: In a large mixing bowl, whisk together the active sourdough starter and warm water until the starter is mostly dissolved and the water looks milky.
Step 2: Add the bread flour and salt to the bowl. Mix by hand or with a sturdy spatula until a shaggy dough forms and no dry flour remains. Cover and let rest for 30 minutes.
Step 3: Perform your first set of stretch and folds. Grab one side of the dough, pull it up high, and fold it over into the center. Rotate the bowl and repeat 4 times. Cover and rest for 30 minutes. Repeat this 3 more times for a total of 4 sets.
Step 4: Allow the dough to rest undisturbed for the remainder of the bulk fermentation (usually 4-6 hours total from the start) until it has increased in volume by about 50% and looks bubbly.
Step 5: Gently turn the dough onto a lightly floured surface. Shape it into a round boule by folding the edges toward the center, then flip it over and use your hands to create surface tension.
Step 6: Place the dough into a floured proofing basket (banneton) or a bowl lined with a floured tea towel. Cover and refrigerate for 12 to 24 hours for a cold proof.
Step 7: Preheat your oven to 450°F (230°C) with a Dutch oven inside for at least 45 minutes. Carefully transfer the cold dough to parchment paper, score the top with a sharp blade, and place it in the Dutch oven. Bake covered for 20 minutes, then uncovered for 20-25 minutes until deep golden brown.