Step 1: In a large bowl, combine the active sourdough starter and warm water. Whisk gently until the starter is mostly dissolved and bubbly. This usually takes about 30 seconds.
Step 2: Add the bread flour and salt to the bowl. Use your hands or a Danish dough whisk to mix until no dry flour remains and a shaggy dough forms. Cover the bowl with a damp cloth or plastic wrap and let it rest for 30 minutes (autolyse).
Step 3: Perform 4 sets of stretch and folds over the next 2-3 hours. For each set, wet your hands slightly, grab a portion of the dough from one side, stretch it upwards, and fold it over to the opposite side. Rotate the bowl and repeat 3-4 times until the dough feels stronger. Cover and rest for 30 minutes between sets. After the last set, cover and let the dough bulk ferment at room temperature (68-75°F / 20-24°C) for 10-16 hours, or until it has increased in volume by about 50-75% and is bubbly.
Step 4: Gently turn the dough out onto a lightly floured surface. Shape it into a round or oval. You can do this by gently pulling the edges of the dough towards the center and then flipping it over, using the surface tension to create a smooth top. Place the shaped dough seam-side up into a floured banneton basket or a bowl lined with a floured tea towel.
Step 5: Cover the banneton/bowl and refrigerate for 2-4 hours (cold proof). This helps develop flavor and makes the dough easier to handle. You can also cold proof overnight for an even deeper flavor.
Step 6: Preheat your oven to 450°F (232°C) with a Dutch oven inside for at least 30 minutes. Carefully remove the hot Dutch oven. Gently invert the dough from the banneton into the hot Dutch oven. Score the top of the dough with a sharp razor blade or knife.
Step 7: Cover the Dutch oven and bake for 30 minutes. Remove the lid and continue to bake for another 10-15 minutes, or until the crust is deeply golden brown and the internal temperature reaches 200-210°F (93-99°C).
Step 8: Carefully remove the bread from the Dutch oven and transfer it to a wire rack to cool completely for at least 1-2 hours before slicing. This is crucial for the crumb to set.