1. Combine Starter and Water: In a large mixing bowl, gently whisk together the active sourdough starter and warm water until the starter is mostly dissolved. The mixture should look milky and slightly bubbly. This step ensures the starter is evenly distributed.

2. Mix in Flour and Salt: Add the all-purpose flour and fine sea salt to the starter mixture. Use a sturdy spoon or your hands to mix until all the flour is incorporated and no dry spots remain. The dough will be shaggy and sticky. Cover the bowl with a damp kitchen towel or plastic wrap and let it rest for 30 minutes. This rest, called autolyse, helps the flour fully hydrate.

3. Perform Stretch and Folds: After the rest, drizzle 1 tablespoon of olive oil over the dough. With wet hands, grab a portion of the dough from one side, stretch it upwards, and fold it over to the opposite side. Rotate the bowl a quarter turn and repeat this stretch and fold motion three more times, completing one full round. The dough will become smoother and more elastic with each fold. Cover and let rest for 30 minutes. Repeat this stretch and fold process three more times, for a total of four sets of folds over 2 hours.

4. Bulk Fermentation: After the final set of folds, cover the bowl and let the dough rise at room temperature for 3-6 hours, or until it has visibly increased in size (about 50-75%) and looks airy and bubbly. The exact time will depend on your kitchen’s temperature and your starter’s strength.

5. Prepare for Baking: Gently transfer the dough to a well-oiled 9×13 inch baking pan (or a similar size). Drizzle another tablespoon of olive oil over the dough. Using your fingertips, gently dimple the entire surface of the dough, pressing down to create characteristic indentations without tearing the dough.

6. Add Toppings and Bake: Sprinkle the chopped fresh rosemary (or other herbs) and flaky sea salt evenly over the dimpled dough. Let the focaccia proof in the pan for another 30-60 minutes while your oven preheats to 425 F (220 C). Bake for 25-30 minutes, or until the focaccia is golden brown on top and cooked through.

7. Cool and Serve: Once baked, carefully remove the focaccia from the oven and transfer it to a wire rack to cool slightly. Drizzle with a little more olive oil if desired before slicing and serving warm.