1. 1. Combine Starter and Water: In a large mixing bowl, gently whisk together your active sourdough starter with the warm water until the starter is mostly dissolved and bubbly. This creates the foundation for your dough.

2. 2. Mix Dry Ingredients: Add the all-purpose flour and fine sea salt to the bowl with the starter mixture. Use a wooden spoon or your hands to mix until a shaggy dough forms and no dry streaks of flour remain. It will be sticky, which is perfectly normal for focaccia dough.

3. 3. First Fermentation (Bulk Rise): Cover the bowl with a damp kitchen towel or plastic wrap. Let the dough rest at room temperature for 30 minutes. After this rest, perform your first “stretch and fold”: lightly wet your hands, grab a portion of the dough from one side, stretch it upwards, and fold it over to the opposite side. Rotate the bowl and repeat this process 3-4 times until the dough feels slightly stronger. Repeat this stretch and fold process every 30-45 minutes for a total of 3-4 rounds over 3-4 hours. The dough should become smoother, more elastic, and increase in volume by about 20-30%.

4. 4. Prepare for Second Rise and Flavor: Lightly grease a 9×13 inch baking pan with 1 tablespoon of olive oil. Transfer the dough into the prepared pan. Drizzle the remaining 2 tablespoons of olive oil over the dough, and sprinkle the minced garlic evenly across the surface. Gently dimple the dough all over with your fingertips, pushing the garlic and oil into the dough.

5. 5. Second Fermentation (Proofing): Cover the pan loosely with plastic wrap or a damp towel. Let the dough proof at room temperature for another 1-2 hours, or until it looks visibly puffy and has expanded by about 50%. You should see small bubbles forming on the surface.

6. 6. Preheat and Bake: While the dough is proofing, preheat your oven to 425 F (220 C). Once the dough is ready, uncover it and sprinkle generously with flaky sea salt and, if using, fresh rosemary sprigs.

7. 7. Bake to Golden Perfection: Bake for 25-30 minutes, or until the focaccia is golden brown on top and sounds hollow when tapped on the bottom. The internal temperature should be around 200-210 F (93-99 C).

8. 8. Cool and Serve: Once baked, 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. This allows the internal structure to set.