If your starter isn't already active, feed it 8-12 hours before you plan to mix your dough. Combine 20g mature starter with 40g water and 40g bread flour. Let it sit at room temperature until it's very active, bubbly, and passes the float test (a small dollop floats in water).

In a large mixing bowl, combine the warm water and the active sourdough starter. Stir gently to break up the starter. Add the bread flour and mix with a spoon or your hand until no dry bits of flour remain and a shaggy dough forms. This is the autolyse stage. Cover the bowl with plastic wrap or a shower cap and let it rest for 30-60 minutes.

After the autolyse, sprinkle the salt over the dough, then drizzle with the olive oil. Use wet hands to incorporate the salt and oil into the dough. Squeeze and fold the dough for about 5 minutes until the ingredients are fully integrated and the dough starts to become more cohesive. It might feel a bit messy at first.

Cover the bowl and let the dough rest for 30 minutes. Perform your first set of stretch and folds: Wet your hands slightly, 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 this process 3 more times until you've stretched and folded all four sides. This should take about 1-2 minutes.

Repeat the stretch and fold process every 30-45 minutes for a total of 3-4 sets. The dough will become progressively stronger, smoother, and more elastic with each set. After the last set, cover the bowl and let the dough continue to bulk ferment undisturbed at room temperature (72-78°F / 22-26°C) for another 1-2 hours, or until it has visibly increased in volume by about 30-50% and feels light and airy.

Lightly flour your work surface. Gently turn the dough out onto the surface. Preshape the dough into a loose rectangle or round. Let it rest for 20-30 minutes, uncovered, to relax the gluten.

To shape for a loaf pan: Gently flatten the dough into a rectangle. Fold the top third down to the middle, then the bottom third up and over, sealing the seam. Turn the dough 90 degrees and repeat: fold the top third down, then the bottom third up, sealing the seam tightly. Gently roll the dough into a log, maintaining tension on the surface. Ensure the seams are pinched completely closed.

Lightly grease your 9x5 inch loaf pan, or line it with parchment paper for easy removal. Carefully transfer the shaped dough log into the prepared loaf pan, seam-side down. Cover the pan tightly with plastic wrap or place it in a large plastic bag. Refrigerate for 12-18 hours (or up to 24 hours). This cold proof develops flavor and makes the dough easier to score.

Preheat your oven to 450°F (232°C) with a rack in the middle, at least 30 minutes before baking. If you like, place an empty broiler pan or cast iron pan on the bottom rack for creating steam.

Carefully remove the loaf pan from the refrigerator. Lightly dust the top of the dough with flour and, using a sharp blade or lame, score the top of the loaf with one long slash down the middle, or several smaller slashes.

Place the loaf pan in the preheated oven. If you're creating steam, pour about 1 cup of hot water into the hot pan on the bottom rack (be careful of the steam!). Immediately close the oven door.

Bake for 20 minutes with steam. Then, carefully remove the steam pan (if used) and reduce the oven temperature to 400°F (200°C). Continue baking for another 20-25 minutes, or until the crust is deeply golden brown and the internal temperature of the bread reaches 200-210°F (93-99°C).

Remove the loaf from the pan immediately and transfer it to a wire rack to cool completely for at least 2-3 hours before slicing. Slicing too early will result in a gummy texture.