Step 1: In a large mixing bowl, combine the bread flour and warm water. Mix until just combined and no dry flour remains. The dough will be shaggy.
Step 2: Cover the bowl with a damp cloth or plastic wrap and let it rest for 30 minutes to 1 hour at room temperature. This is the autolyse phase.
Step 3: Add the active sourdough starter directly to the dough. Using wet hands, dimple the starter into the dough until fully incorporated.
Step 4: Sprinkle the salt over the dough. Fold the dough over on itself several times, squeezing gently to distribute the salt evenly.
Step 5: Cover and let rest for 30 minutes.
Step 6: Perform the first set of stretch and folds. With wet hands, grab a portion of the dough from one side, stretch it upwards gently, and fold it over to the opposite side of the bowl. Rotate the bowl a quarter turn and repeat this process 3-4 more times.
Step 7: Cover and let rest for 30 minutes.
Step 8: Repeat the stretch and fold process (Step 6) three more times, with 30-minute rest periods in between each set. The total bulk fermentation time will be around 3-4 hours, or until the dough increases in volume by about 20-30%.
Step 9: Gently tip the dough out onto a lightly floured surface. Using a bench scraper, gently shape the dough into a loose round or rectangle, creating some surface tension.
Step 10: Cover with a damp cloth and let rest for 20-30 minutes (bench rest).
Step 11: Lightly flour a banneton (proofing basket) or a bowl lined with a floured tea towel.
Step 12: Gently flip the dough over so the seam is facing up. Shape the dough into a tight boule (round) or batard (oval), creating significant surface tension. Place into the prepared banneton, seam-side up.
Step 13: Cover the banneton with plastic wrap or place it inside a large plastic bag. Refrigerate for at least 12 hours, and up to 24 hours (cold proof).
Step 14: About 45-60 minutes before baking, preheat your oven to 475°F (245°C) with a Dutch oven inside. Make sure it's fully preheated.
Step 15: Carefully remove the hot Dutch oven from the oven. Gently invert the cold dough from the banneton directly into the hot Dutch oven.
Step 16: Using a sharp razor blade or lame, score the top of the dough.
Step 17: Cover the Dutch oven and bake for 20 minutes.
Step 18: Remove the lid from the Dutch oven, reduce the oven temperature to 450°F (230°C), and continue to bake for another 20-25 minutes, or until the crust is deeply golden brown and the internal temperature reaches 200-210°F (93-99°C).
Step 19: Carefully remove the loaf from the Dutch oven and place it on a wire rack to cool completely for at least 1-2 hours before slicing.