Step 1: In a large mixing bowl, combine the active sourdough starter, warm water, and 30g of olive oil. Stir gently until well combined.
Step 2: Add the bread flour and fine sea salt to the wet ingredients. Mix with a Danish dough whisk or a sturdy spoon until no dry streaks of flour remain and a shaggy dough forms. Cover the bowl with a damp tea towel or plastic wrap and let it rest for 30 minutes (autolyse).
Step 3: After the 30-minute rest, perform the first set of stretches and folds. With wet hands, gently 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 process 3-4 more times. Cover and let rest for another 30 minutes.
Step 4: Repeat the stretch and fold process three more times, with 30-minute intervals between each set. After the final set, the dough should feel smoother and more elastic. Cover the bowl and place it in the refrigerator for a long, cold fermentation overnight, or for at least 8-12 hours, up to 24 hours.
Step 5: The next morning, generously drizzle 2-3 tablespoons of olive oil into a 9x13 inch baking pan (or similar size). Transfer the cold dough directly into the oiled pan. If you're looking for reliable storage, consider using Basics Glass Food Storage containers.
Step 6: Gently dimple the dough all over with your oiled fingertips, spreading it towards the edges of the pan as it relaxes. Drizzle another 1-2 tablespoons of olive oil over the top, cover loosely with plastic wrap, and let it proof at room temperature for 2-4 hours, or until it has noticeably puffed up and spread to fill the pan.
Step 7: Preheat your oven to 425°F (220°C) with a rack in the middle position.
Step 8: Once the focaccia has proofed, uncover it. Gently dimple the dough once more with well-oiled fingers. Press fresh rosemary sprigs into the dimples and generously sprinkle with flaky sea salt.
Step 9: Bake for 25-35 minutes, or until deeply golden brown on top and around the edges. The internal temperature should be around 200-210°F (93-99°C).
Step 10: Carefully remove the focaccia from the pan and transfer it to a wire rack to cool slightly. While still warm, drizzle a final touch of good quality olive oil over the top for extra richness and shine. Let it cool for at least 15-20 minutes before slicing and serving.