Step 1: In a large bowl, whisk together 100g of active starter and 350g of water until the starter is mostly dissolved and the water looks milky.
Step 2: Add 500g of bread flour to the water mixture and stir with a spatula or your hands until no dry flour remains. Cover with a damp cloth and let rest (autolyse) for 30 to 60 minutes.
Step 3: Sprinkle 10g of salt over the dough. Using wet hands, squeeze the salt into the dough and perform a set of 'stretch and folds' to incorporate it thoroughly.
Step 4: Perform 4 sets of stretch and folds every 30 minutes over the next 2 hours. To fold, pull the edge of the dough up and over towards the center, rotating the bowl 90 degrees each time.
Step 5: Let the dough rest undisturbed for the remainder of the bulk fermentation (usually 2-4 hours) until it has grown about 50% in size and looks bubbly.
Step 6: Gently turn the dough onto a lightly floured surface. Fold the edges into the center to create a round shape, then flip it over and use your hands to create tension on the surface.
Step 7: Place the dough upside down into a floured proofing basket (banneton) or a bowl lined with a floured towel. Cover and refrigerate for 12 to 18 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.
Step 9: Carefully flip the cold dough onto a piece of parchment paper. Score the top with a sharp blade or lame. Use the parchment to lift the dough into the hot Dutch oven.
Step 10: Bake with the lid on for 20 minutes. Remove the lid and continue baking for 20-25 minutes until the sourdough bread is deep golden brown and sounds hollow when tapped.