In a large mixing bowl, combine the active sourdough starter and the warm water. Whisk gently with a fork or your fingers until the starter is mostly dispersed in the water. This is your autolyse liquid.

Add the bread flour to the bowl with the starter mixture. Mix with your hands or a sturdy spatula until just combined and no dry flour remains. The dough will be shaggy and rough. Cover the bowl with a damp cloth or plastic wrap and let it rest for 30 minutes. This resting period is called autolyse and helps the flour hydrate, making gluten development easier.

After the autolyse, sprinkle the fine sea salt over the dough. With wet hands, pinch and fold the salt into the dough, ensuring it's evenly distributed. This process incorporates the salt and begins developing the gluten structure. Perform a few stretch and folds within the bowl to help integrate the salt.

Perform the first set of stretch and folds. Wet your hands slightly. Grab a portion of the dough from one side, stretch it upwards gently, and fold it over the center. Rotate the bowl 90 degrees and repeat this process three more times, for a total of four folds. This builds strength in the dough. Cover the bowl and let it rest for 30 minutes.

Repeat the stretch and fold process (Step 4) every 30 minutes for a total of 3-4 sets over the next 1.5 to 2 hours. The dough will gradually become smoother, more elastic, and increase in volume. It should start to feel cohesive and hold its shape better.

Once the dough has developed sufficient strength (it should pass the 'windowpane test' - a small piece can be stretched thin enough to see light through without tearing), it's time for bulk fermentation. Cover the bowl and let the dough rest at room temperature for another 2-4 hours, or until it has increased in volume by about 30-50% and shows signs of aeration with small bubbles on the surface. The exact time will depend on your room temperature and starter activity.

Lightly dust your work surface with a small amount of flour. Gently scrape the dough out of the bowl onto the floured surface. Be careful not to deflate it too much. Perform a gentle pre-shape: lightly shape the dough into a round by pulling the edges towards the center and then cupping your hands around it to create surface tension. Let it rest, uncovered, on the counter for 20-30 minutes. This allows the gluten to relax before the final shaping.

Prepare your proofing basket (banneton) by generously dusting it with rice flour or semolina flour. This prevents sticking. If you don't have a banneton, a bowl lined with a floured tea towel will work.

Perform the final shaping. Lightly flour the top of the rested dough. Flip it over so the seam side is up. Gently shape the dough into your desired final form (boule or bâtard) by folding the edges towards the center and creating surface tension. Be firm but gentle to avoid degassing the dough. Place the shaped dough seam-side up into the prepared proofing basket.

Cover the proofing basket with plastic wrap or place it inside a plastic bag. Refrigerate the dough for a cold proof of 12-24 hours. This slow fermentation develops flavor and makes the dough easier to score.

When ready to bake, preheat your oven with a Dutch oven (or baking stone/steel with a steam pan) inside to 240°C (475°F) for at least 45 minutes. Ensure the Dutch oven is fully heated.

Carefully remove the hot Dutch oven from the oven. Take the dough out of the refrigerator. Invert the dough onto a piece of parchment paper. Gently score the top of the loaf with a lame or sharp razor blade. A simple cross or a decorative pattern works well. Make a cut about 1.25cm (0.5 inch) deep.

Using the parchment paper as a sling, carefully lower the dough into the preheated Dutch oven. Place the lid back on the Dutch oven.

Bake with the lid on for 25 minutes at 240°C (475°F). The trapped steam will help the bread rise and develop a beautiful crust.

After 25 minutes, carefully remove the lid from the Dutch oven. Reduce the oven temperature to 220°C (425°F). Continue to bake for another 20-25 minutes, or until the crust is a deep golden brown and the internal temperature of the bread reaches 96-100°C (205-210°F) when measured with an instant-read thermometer.

Once baked, carefully remove the bread from the Dutch oven and place it on a wire rack to cool completely. This is crucial for the crumb structure to set. Resist the urge to cut into it while it's hot!