In a large mixing bowl, combine the bread flour and 300g of the warm water. Mix with a spoon or your hand until no dry spots remain and a shaggy dough forms. Cover the bowl with a damp towel or plastic wrap and let it rest for 30 minutes at room temperature. This step helps the flour hydrate and gluten develop.

After the autolyse, add the active sourdough starter and the remaining 40g of water to the dough. Squeeze the dough gently with your hand to incorporate the starter and water. Once mostly mixed, add the salt and continue to gently squeeze and fold until the salt is fully integrated and the dough feels slightly more cohesive.

Lightly oil your hands and the bowl. Perform the first set of stretch and folds: 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 and repeat this motion 3-4 times until you've worked your way around the dough. Cover and let rest for 30 minutes.

Repeat the stretch and fold process three more times, with 30-minute intervals between each set. After the fourth set (approximately 2 hours total bulk fermentation), the dough should feel stronger, more elastic, and have some air bubbles. Cover the bowl and let it rest for a final 30-45 minutes, or until it has visibly increased in volume by about 20-30%.

Generously dust your work surface with semolina or rice flour. Carefully invert the dough onto the floured surface. Gently stretch the dough into a roughly rectangular shape (about 12x8 inches), being careful not to deflate it. Cut the dough into 2-3 equal pieces using a dough scraper.

For each piece, gently stretch it lengthwise into your desired ciabatta loaf shape, maintaining a relatively flat profile. Avoid overworking. Generously dust a sheet of parchment paper with semolina or rice flour. Carefully transfer the shaped ciabatta loaves onto the parchment paper, ensuring they have enough space between them.

Cover the ciabatta loaves loosely with a light towel or plastic wrap. Let them proof at room temperature for 30-45 minutes. During this time, preheat your oven to 475°F (245°C) with your baking steel or pizza stone inside for at least 30 minutes to ensure it's thoroughly hot. Place an empty cast iron pan or oven-safe tray on the bottom rack for steam.

Carefully slide the parchment paper with the ciabatta onto the preheated baking steel/stone. Pour about 1/2 cup of hot water into the preheated pan on the bottom rack (this creates steam for a crispy crust). Immediately close the oven door.

Bake for 15 minutes with steam, then carefully open the oven door to release the steam, remove the water pan, and lower the temperature to 450°F (230°C). Continue baking for another 10-15 minutes, or until the ciabatta is golden brown and sounds hollow when tapped on the bottom.

Transfer the ciabatta loaves to a wire rack to cool completely before slicing. This is crucial for the crumb structure to set.