Step 1: In a large mixing bowl, combine the active sourdough starter, warm water, and 30g of olive oil. Stir gently until the starter is mostly dissolved.
Step 2: Add the all-purpose flour and fine sea salt to the wet ingredients. Mix with a sturdy spoon or your hands until a shaggy dough forms and no dry flour remains.
Step 3: Cover the bowl with a damp cloth or plastic wrap and let the dough rest for 30 minutes (autolyse).
Step 4: Perform the first set of stretch and folds. With wet hands, 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 a quarter turn and repeat 3-4 more times.
Step 5: Cover and let the dough rest for another 30 minutes. Repeat the stretch and fold process three more times, with 30-minute intervals between each set. After the fourth set, the dough should be smoother and more elastic.
Step 6: Cover the bowl tightly and place it in the refrigerator for a cold bulk fermentation of 12-24 hours. This develops flavor and simplifies handling.
Step 7: The next day, generously grease a 9x13 inch baking pan with olive oil (or line with parchment and then grease).
Step 8: Gently scrape the cold dough from the bowl into the prepared baking pan. Drizzle a little more olive oil over the top.
Step 9: With oiled fingertips, gently dimple the dough all over, pressing down to the bottom of the pan without tearing. Ensure even dimpling across the entire surface.
Step 10: Evenly sprinkle the chopped fresh rosemary and flaky sea salt over the dimpled dough. Gently press them into the dough.
Step 11: Cover the pan loosely with plastic wrap or a clean kitchen towel. Let the focaccia proof at room temperature for 2-4 hours, or until it looks visibly puffy and has increased by about 50-75% in volume.
Step 12: Preheat your oven to 425°F (220°C) with a rack in the middle position during the last hour of proofing.
Step 13: Once proofed, uncover the focaccia and bake for 25-35 minutes, or until deeply golden brown and cooked through.
Step 14: Remove from the oven. For a crispier bottom, immediately transfer the focaccia from the pan to a wire rack. Drizzle with a final touch of fresh olive oil while warm.
Step 15: Let it cool for at least 15-20 minutes before slicing and serving. Enjoy your homemade Artisanal Sourdough Focaccia with Rosemary!