Step 1: In a large mixing bowl, whisk together the active sourdough starter and warm water until the starter is mostly dissolved and the water looks milky.
Step 2: Add the bread flour and sea salt to the bowl. Mix by hand or with a sturdy spoon until a shaggy, sticky dough forms and no dry flour remains.
Step 3: Cover the bowl with a damp cloth and let the dough rest for 30 minutes. Perform 4 sets of ‘stretch and folds’ every 30 minutes over the next 2 hours to build dough strength.
Step 4: Allow the dough to sit at room temperature for bulk fermentation until it has increased in volume by about 50-75% and appears bubbly on the surface.
Step 5: Gently shape the dough into a round boule on a floured surface. Place it in a proofing basket and let it rest in the refrigerator for 12 to 18 hours (cold proof).
Step 6: Preheat your oven to 450°F with a Dutch oven inside. Score the dough, place it in the preheated pot, and bake with the lid on for 20 minutes, then lid off for 25 minutes until golden brown.