1. 1. Activate Your Starter and Mix the Dough: In a large mixing bowl, combine your active sourdough starter, warm water, and granulated sugar. Stir until the starter is mostly dissolved. Add the all-purpose flour and 2 teaspoons of fine sea salt. Mix with a wooden spoon or your hands until a shaggy, cohesive dough forms. It will be sticky, but don’t worry about kneading it perfectly at this stage.

2. 2. First Rise (Bulk Fermentation): Cover the bowl with a damp kitchen towel or plastic wrap. Let the dough rest at room temperature (ideally around 70-75 F) for 2 to 3 hours. During this time, the dough should visibly increase in size and become slightly bubbly. The exact time will depend on the strength of your starter and your room temperature.

3. 3. Prepare the Baking Pan: While the dough is rising, generously grease a 9×13-inch baking pan with about 2 tablespoons of olive oil. Make sure the oil coats the bottom and sides well; this prevents sticking and contributes to a crispy bottom crust.

4. 4. Transfer and Dimple the Dough: Once the dough has risen, gently pour it into the prepared baking pan. With oiled hands, gently spread the dough to fill the pan, being careful not to deflate it too much. Use your fingertips to create deep indentations (dimples) all over the surface of the dough.

5. 5. Add Toppings and Final Rise: Drizzle the remaining olive oil generously over the dimpled dough. Scatter the fresh rosemary leaves evenly over the top, and then sprinkle with a good pinch of flaky sea salt. Cover the pan loosely with plastic wrap or a damp towel and let it rise again for another 30-45 minutes while your oven preheats.

6. 6. Preheat and Bake: Preheat your oven to 400 F (200 C). Once the oven is hot and the focaccia has had its final rise, remove the cover and place the pan in the preheated oven. Bake for 25 to 30 minutes, or until the focaccia is golden brown on top and sounds hollow when tapped on the bottom.

7. 7. Cool and Serve: Once baked, remove the focaccia from the oven and carefully transfer it to a wire rack to cool slightly. This prevents the bottom from becoming soggy. Slice into squares or strips and serve warm.