Step 1: In a large mixing bowl, combine 350g active sourdough starter, 500g bread flour, and 350g lukewarm water. Mix until no dry flour remains and the dough is shaggy and sticky. Cover with a damp cloth or plastic wrap and let it rest for 30 minutes to 1 hour (autolyse).
Step 2: Sprinkle 10g fine sea salt over the dough. With wet hands, gently incorporate the salt by squeezing and folding the dough for about 5-10 minutes until it becomes smoother and more elastic.
Step 3: Cover the bowl and let it rest at room temperature (ideally 75-78°F / 24-25°C). Perform 4-6 sets of ‘stretch and folds’ every 30-45 minutes over the next 3-5 hours. The dough should become strong and bubbly.
Step 4: Gently turn the dough out onto a lightly floured surface. Shape it into a loose round or rectangle. Let it rest, uncovered, for 20 minutes to relax the gluten.
Step 5: Lightly flour your hands and work surface. Gently flatten the dough, then fold the bottom third up, and the top third down. Turn the dough over and gently pull it towards you to create surface tension. Place the shaped dough seam-side up into a floured banneton or a bowl lined with a floured tea towel.
Step 6: Cover the banneton and place it in the refrigerator for 8-16 hours for cold proofing. This develops flavor and makes the dough easier to handle.
Step 7: Preheat your oven to 450°F (232°C) with a Dutch oven inside for 30-45 minutes. Carefully invert the cold dough into the hot Dutch oven. Score the top of the dough with a sharp blade. Cover and bake for 30 minutes.
Step 8: Remove the lid from the Dutch oven and continue baking for another 15-25 minutes, or until the crust is deep golden brown and the internal temperature is 200-210°F (93-99°C).
Step 9: Transfer the baked sourdough loaf to a wire rack and let it cool completely for at least 1-2 hours before slicing to allow the internal structure to set.