Step 1: In a large glass bowl, whisk together the active sourdough starter and warm water until the starter is mostly dissolved and the water is milky.
Step 2: Add the bread flour to the water mixture and stir with a sturdy spatula or your hands until a shaggy dough forms and no dry flour remains. Cover with a damp cloth and let rest (autolyse) for 30 to 60 minutes.
Step 3: Sprinkle the sea salt over the dough. Wet your hands and squeeze the salt into the dough, then perform a few folds to ensure it is evenly distributed. Cover and let rest for 30 minutes.
Step 4: Perform the first set of stretch and folds. Grab one side of the dough, pull it up high, and fold it over to the center. Rotate the bowl and repeat 4 times until you have gone all the way around. Repeat this process every 30 minutes for a total of 4 sets (2 hours).
Step 5: Cover the bowl and allow the dough to sit undisturbed for the remainder of the bulk fermentation (usually another 2-4 hours depending on kitchen temperature) until it has grown by about 50% and looks bubbly.
Step 6: Gently turn the dough onto a lightly floured surface. Fold the edges into the center to create a round ball, then flip it over. Use your hands to tension the surface by pulling the dough toward you on the counter. Let rest for 20 minutes.
Step 7: Place the dough upside down into a floured banneton or a bowl lined with a floured kitchen towel. Cover and place in the refrigerator for 8 to 24 hours for a cold proof.
Step 8: Preheat your oven to 450°F (230°C) with a Dutch oven inside for at least 45 minutes. Carefully transfer the cold dough onto parchment paper, score the top with a sharp blade, and place it into the hot Dutch oven. Bake with the lid on for 20 minutes, then remove the lid and bake for another 20-25 minutes until deep golden brown.