1. Combine the Wet Ingredients: In a large mixing bowl, gently whisk together your active sourdough starter and the warm water until the starter is mostly dissolved. The mixture should look milky and slightly bubbly.

2. Add Dry Ingredients and Mix: Stir in the all-purpose flour and fine sea salt until just combined. The dough will be shaggy and sticky. Cover the bowl with a clean kitchen towel or plastic wrap and let it rest for 30 minutes. This resting period, known as autolyse, helps the flour absorb the water and develop gluten.

3. Perform the First Stretch and Fold: After the rest, drizzle about 1 tablespoon of olive oil over the dough. With wet hands, grab a section of the dough from the edge, stretch it upwards, and fold it over to the center. Rotate the bowl a quarter turn and repeat this process 3-4 more times until the dough feels slightly smoother and more elastic. Cover and let it rest for another 30 minutes.

4. Repeat Stretch and Folds: Repeat the stretch and fold process two more times, with 30-minute rests in between each set. After the third set, the dough should be noticeably smoother, more cohesive, and have some visible air bubbles.

5. Bulk Fermentation: Drizzle a little olive oil into a clean, large bowl or a 9×13 inch baking pan. Transfer the dough to the oiled bowl/pan, turning it once to coat. Cover tightly with plastic wrap and let it rise at room temperature for 4-6 hours, or until it has almost doubled in size and is very bubbly. Alternatively, you can refrigerate it overnight for a slower, colder proof.

6. Dimple and Top the Dough: Preheat your oven to 425 F (220 C). If using a bowl for the bulk fermentation, gently transfer the risen dough to a well-oiled 9×13 inch baking pan. Drizzle generously with olive oil (about 2-3 tablespoons). Using your fingertips, gently dimple the entire surface of the dough, pressing almost all the way down to the bottom of the pan. Sprinkle with dried rosemary (if using) and a generous amount of flaky sea salt.

7. Bake to Golden Perfection: Place the focaccia in the preheated oven and bake for 25-30 minutes, or until the top is golden brown and the edges are crispy. The internal temperature should register around 200-210 F (93-99 C) if you have a thermometer.

8. Cool and Serve: Once baked, carefully remove the focaccia from the oven. Let it cool in the pan for a few minutes before transferring it to a wire rack to cool completely. Slicing it while still warm is perfectly fine, but allowing it to cool slightly helps set the texture.