Step 1: In a large bowl, combine 100g of your active, bubbly sourdough starter with 350g of filtered water. Stir until the starter is mostly dissolved.
Step 2: Add 500g of bread flour to the bowl. Mix with your hand or a sturdy spoon until no dry flour remains and a shaggy dough forms. Don't worry about developing gluten at this stage; just ensure all flour is hydrated.
Step 3: Cover the bowl with a damp cloth or plastic wrap and let it rest for 30 minutes to 1 hour (autolyse).
Step 4: After the autolyse, sprinkle 10g of fine sea salt over the dough. Using wet hands, dimple the salt into the dough, then gently fold and squeeze the dough for 2-3 minutes to fully incorporate the salt.
Step 5: Cover the bowl and let the dough rest for 30 minutes.
Step 6: Perform the first set of 'stretch and folds'. With wet hands, gently grab a portion of the dough from one side, stretch it upwards, and fold it over to the opposite side of the bowl. Rotate the bowl a quarter turn and repeat, performing 4-6 stretches and folds in total.
Step 7: Cover and rest for 30 minutes.
Step 8: Repeat Step 6 (stretch and folds) every 30-45 minutes for a total of 4-5 sets. The dough should increase in volume by about 30-50%.
Step 9: Gently turn the dough out onto a lightly floured surface. Lightly flour the top of the dough. Using a bench scraper or your hands, gently form the dough into a loose round or rectangle. Allow it to rest uncovered for 20-30 minutes.
Step 10: Lightly flour the dough and your hands. Flip the dough over so the floured side is down. For a round loaf (boule): Gently pull the top edge of the dough up and fold it down towards the center. Rotate the dough 180 degrees, pull the new top edge up, and fold it down to overlap the previous fold. Then, gently pull the left side over to the right, and the right side over to the left, creating a tight package. Flip the dough seam-side up and gently cup it with your hands, rotating it on the counter to create surface tension.
Step 11: Generously flour a banneton with rice flour. Carefully transfer the shaped dough, seam-side up, into the banneton.
Step 12: Cover the banneton with a plastic bag or a shower cap and place it in the refrigerator for 12-18 hours (cold proof).
Step 13: Preheat your oven to 500°F (260°C) with a Dutch oven inside for at least 45 minutes to 1 hour.
Step 14: Carefully remove the hot Dutch oven from the oven. Gently invert the cold dough from the banneton onto a piece of parchment paper.
Step 15: Using a sharp razor blade (lame), score the top of the dough.
Step 16: Carefully lift the dough with the parchment paper and transfer it into the hot Dutch oven. Place the lid on, and bake for 20 minutes.
Step 17: After 20 minutes, remove the lid. Reduce the oven temperature to 450°F (232°C) and bake for another 25-30 minutes, or until the crust is deep golden brown and the internal temperature reaches 200-210°F (93-99°C).
Step 18: Carefully remove the loaf from the Dutch oven and transfer it to a wire rack to cool completely for at least 2-3 hours before slicing.