Step 1: Feed your starter the night before you plan to mix your dough. Combine 20g starter, 40g unbleached bread flour, and 40g warm water in a clean jar. Stir well, cover loosely, and let it sit at room temperature (around 70-75°F / 21-24°C) overnight. It should be bubbly and active, having at least doubled in size, by the time you're ready to mix the dough the next morning.
Step 2: In a large mixing bowl, combine 350g warm water and 450g bread flour. Mix until just combined and no dry spots of flour remain. Cover the bowl and let it rest for 30-60 minutes (autolyse).
Step 3: After the autolyse, add 100g active sourdough starter and 10g fine sea salt to the dough. Using wet hands, gently mix and squeeze the dough to fully incorporate the starter and salt until evenly distributed.
Step 4: Perform the first set of stretch and folds. Wet your hands slightly. Grab a section of the dough, stretch it upwards, and fold it over onto the center of the dough. Rotate the bowl a quarter turn and repeat this process 3-4 more times. Cover the bowl.
Step 5: Over the next 2.5-3.5 hours, perform 3-4 more sets of stretch and folds, spaced 30-45 minutes apart. The dough will become stronger and smoother with each set.
Step 6: After the last set of stretch and folds, cover the bowl and let the dough rest undisturbed for the remainder of the bulk fermentation period. The dough should look visibly gassy, slightly domed, and have increased in volume by about 20-30%.
Step 7: Lightly flour your work surface. Gently turn the dough out onto the floured surface. Using a bench scraper, gently form the dough into a loose ball, creating surface tension. Let it rest, uncovered, for 20-30 minutes (pre-shape).
Step 8: Lightly flour your banneton (or a bowl lined with a floured kitchen towel). Gently transfer the pre-shaped dough to your floured surface. For a round loaf, gently fold the top third of the dough down, then the bottom third up, then gently roll it into a tight ball, creating maximum surface tension. Place the shaped dough seam-side up in your prepared banneton (final shape).
Step 9: Cover the banneton with a plastic bag or wrap and place it in the refrigerator for 12-18 hours (cold proof).
Step 10: About 45-60 minutes before baking, place your Dutch oven (with its lid) in your oven and preheat to 475°F (245°C).
Step 11: Carefully remove the hot Dutch oven. Gently invert the cold dough from the banneton directly into the hot Dutch oven. Score the top of the dough with a sharp razor blade or lame. Place the lid back on the Dutch oven.
Step 12: Bake with the lid on for 30 minutes. Then, remove the lid and reduce the oven temperature to 450°F (230°C). Continue to bake for another 15-20 minutes, or until the crust is deeply golden brown and the internal temperature reaches 200-210°F (93-99°C).
Step 13: Carefully remove the bread from the Dutch oven and place it on a wire rack to cool for at least 1-2 hours before slicing.