Combine 200 g active sourdough starter, 350 g warm water, and 500 g bread flour in a large mixing bowl; stir until a rough dough forms (about 2 minutes). Optionally cover and let autolyse for 30 minutes.

Dissolve 10 g sea salt in a tablespoon of water, add to the dough, and knead by squeezing and folding for 3 to 4 minutes until fully incorporated.

Perform the first set of stretch and folds by wetting your hands, lifting one side of the dough, folding it over, rotating the bowl 90°, and repeating on all sides (about 1 minute).

Over the next 2 hours, perform three more sets of stretch and folds every 30 minutes until the dough is smooth, puffy, and has increased in volume by about 30%.

After the final fold, let the dough rest at room temperature until bulk fermentation is complete (about 30 additional minutes); the dough should feel airy and jiggly.

Turn the dough onto a lightly floured surface and pre-shape it into a round using surface tension; rest uncovered for 20 to 30 minutes.

Final shape the dough by folding the sides toward the center and rolling to create a tight surface tension, then place seam-side up in a floured banneton.

Cover the banneton loosely and proof at room temperature for 1 to 2 hours until the dough is puffy and springs back slowly when poked.

Meanwhile, place a Dutch oven in your oven and preheat to 500°F (260°C) for at least 30 minutes.

Carefully transfer the dough onto parchment, score the top with a ¼-inch incision at a 45° angle using a bread lame or sharp knife, then place dough and parchment into the hot Dutch oven, cover, and reduce oven temperature to 450°F (232°C).

Bake covered for 20 minutes, remove the lid, and bake uncovered for an additional 15 to 20 minutes until the crust is deep golden brown.

Remove the loaf from the Dutch oven and cool completely on a wire rack for at least 30 minutes before slicing to set the crumb.