Step 1: In a large glass bowl, whisk together 100g of active starter and 350g of lukewarm water until the starter is mostly dissolved and the water looks milky.
Step 2: Add 500g of bread flour to the bowl and mix by hand or with a sturdy spatula until no dry flour remains. Cover with a damp cloth and let rest (autolyse) for 45 minutes.
Step 3: Sprinkle 10g of sea salt over the dough. Wet your hands and squeeze the salt into the dough, then perform a series of ‘stretch and folds’ for 2 minutes to incorporate. Cover and rest.
Step 4: Perform 4 sets of stretch and folds every 30 minutes. To do this, grab one side of the dough, pull it up high, and fold it over into the center. Rotate the bowl and repeat 4 times per set.
Step 5: Let the dough rest at room temperature (70-75°F) for the bulk fermentation. This usually takes 4-6 hours, or until the dough has increased in volume by about 50% and shows small bubbles on the surface.
Step 6: Gently turn the dough onto a lightly floured surface. Fold the edges toward the center to create a round shape. Let rest for 15 minutes (bench rest).
Step 7: Final shaping: Flip the dough over and tension it against the counter to create a tight skin. Place upside down into a floured banneton or cloth-lined bowl.
Step 8: Cover the basket with plastic or place in a bag and refrigerate for 12 to 18 hours. This is the cold proof that develops the sourdough bread flavor.
Step 9: Preheat your oven to 500°F (260°C) with a Dutch oven inside for at least 45 minutes. Carefully transfer the cold dough onto parchment paper, score the top with a razor blade, and lower it into the hot Dutch oven.
Step 10: Bake with the lid on for 20 minutes. Reduce the oven temperature to 450°F (230°C), remove the lid, and bake for another 20-25 minutes until the crust is deep golden brown. Let cool on a wire rack for at least 2 hours before slicing.