Step 1: In a large bowl, combine 400g bread flour and 300g non-chlorinated water. Mix until just combined and no dry flour remains. 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, add 100g active sourdough starter to the flour mixture. Mix gently with your hand, squeezing the dough between your fingers, until the starter is fully incorporated. Cover again and let rest for 30 minutes.
Step 3: Now, add 8g fine sea salt. Wet your hands slightly and gently fold the dough over itself for a few minutes, working the salt in. Cover and rest for 30 minutes.
Step 4: Over the next 3-4 hours, perform 4-6 sets of 'stretch and folds' every 30 minutes. With wet hands, gently grab a portion of the dough from one side, stretch it upwards, and fold it over to the opposite side. Rotate the bowl and repeat 3-4 times per set. The dough should become smoother and more elastic.
Step 5: After the stretch and folds, let the dough rest, covered, at room temperature for another 8-14 hours, or until it has roughly doubled in size and shows signs of activity.
Step 6: Gently turn your dough out onto a lightly floured surface. Gently pre-shape the dough into a loose round or rectangle. Let it rest, uncovered, for 20-30 minutes.
Step 7: For the final shape, gently shape your dough into a tight boule or batard, creating surface tension. Place the shaped dough seam-side up into a floured banneton basket or a bowl lined with a floured towel.
Step 8: Cover the banneton with plastic wrap or a plastic bag and refrigerate for 12-24 hours for cold proofing.
Step 9: Preheat your oven to 475°F (245°C) with a Dutch oven inside for at least 45 minutes to 1 hour.
Step 10: Carefully remove the hot Dutch oven. Gently invert the cold dough into the hot Dutch oven. Score the top of your sourdough with a sharp razor blade. Bake with the lid on for 25 minutes.
Step 11: Remove the lid and reduce the oven temperature to 450°F (230°C). Continue baking for another 20-25 minutes, or until the crust is a deep golden brown and the internal temperature reaches 200-210°F (93-99°C).
Step 12: Carefully remove the sourdough bread from the Dutch oven and place it on a wire rack to cool completely for at least 1-2 hours before slicing.