Step 1: In a large mixing bowl, combine the active sourdough starter, warm water, bread flour, and fine sea salt. Mix with a Danish dough whisk or your hands until no dry spots remain and a shaggy dough forms. Cover the bowl with a damp cloth or plastic wrap and let it rest for 30 minutes (autolyse).
Step 2: Perform the first set of stretch and folds. With wet hands, gently pull a section of the dough up from the edge of the bowl, stretch it, and fold it over the center. Rotate the bowl a quarter turn and repeat, completing 4-6 stretches and folds around the bowl.
Step 3: Cover the bowl and let the dough rest for 30 minutes. Repeat the stretch and fold process 3 more times, with 30-minute intervals between each set. The dough should become smoother and more elastic with each set.
Step 4: After the final stretch and fold, cover the bowl and let the dough bulk ferment at room temperature for 3-4 hours, or until it has increased in volume by about 20-30% and shows visible bubbles on the surface.
Step 5: Generously oil a 9×13 inch baking pan (or similar sized cast iron pan) with 3-4 tablespoons of olive oil. Gently transfer the dough into the prepared pan, being careful not to deflate it. Gently spread the dough to roughly fill the pan. Drizzle another 1-2 tablespoons of olive oil over the top, cover the pan tightly with plastic wrap, and refrigerate overnight (8-16 hours) for a cold proof.
Step 6: The next day, remove the focaccia from the refrigerator and let it sit at room temperature for 2-3 hours to warm up and continue its final proof. The dough should puff up slightly and be noticeably jiggly. Preheat your oven to 450°F (230°C) with a rack in the middle.
Step 7: Once the dough is at room temperature and puffy, drizzle a little more olive oil over the top. Using well-oiled fingers, gently dimple the entire surface of the dough, pressing all the way down to the bottom of the pan to create characteristic focaccia divots.
Step 8: Arrange the fresh rosemary sprigs (or sprinkle chopped rosemary) and generously sprinkle flaky sea salt over the dimpled dough.
Step 9: Bake for 25-35 minutes, or until the focaccia is golden brown on top and the edges are crispy. Tap the bottom; it should sound hollow when done.
Step 10: Carefully remove the focaccia from the pan and transfer it to a wire rack to cool for at least 15-20 minutes before slicing and serving. Enjoy your delicious Artisanal Sourdough Focaccia with Rosemary warm!