Step 1: In a large bowl, whisk together the active sourdough starter and warm water until the starter is mostly dissolved.
Step 2: Add the bread flour and salt to the bowl. Mix by hand or with a spatula until no dry flour remains and a shaggy dough forms. Let rest for 30 minutes.
Step 3: Perform your first set of stretch and folds by pulling the edge of the dough up and over towards the center. Rotate the bowl and repeat 4 times. Repeat this process every 30 minutes for a total of 4 sets.
Step 4: Cover the dough and let it undergo bulk fermentation at room temperature for 4-6 hours, or until it has grown by about 50% and looks jiggly.
Step 5: Generously grease a 9x13 inch baking pan with olive oil. Transfer the dough to the pan, flip it once to coat in oil, and cover. Let it rest in the fridge overnight (12-24 hours).
Step 6: Remove the dough from the fridge 2 hours before baking. Let it come to room temperature and fill the corners of the pan naturally.
Step 7: Preheat your oven to 425°F (220°C). Drizzle the top of the dough with the remaining olive oil.
Step 8: Use your fingertips to deeply dimple the dough all over, pressing down to the bottom of the pan to create large bubbles.
Step 9: Sprinkle with fresh rosemary and flaky sea salt. Bake for 20-25 minutes until the top is deeply golden brown.
Step 10: Allow the focaccia to cool in the pan for 5 minutes before transferring to a wire rack. Slice and serve warm.