Step 1: In a large bowl, whisk together the active sourdough starter and warm water until the starter is mostly dissolved and the water is milky.
Step 2: Add the bread flour and sea salt to the liquid mixture. Use a sturdy spatula or your hands to mix until a shaggy dough forms and no dry flour remains.
Step 3: Let the dough rest for 30 minutes. Perform 4 sets of ‘stretch and folds’ every 30 minutes. To do this, grab one side of the dough, pull it upward, and fold it over the center. Rotate the bowl and repeat 4 times per set.
Step 4: After the final set of folds, cover the bowl and let it sit at room temperature for 2-4 hours, then transfer to the refrigerator for an overnight cold ferment (12-18 hours).
Step 5: Generously oil a 9×13 inch baking pan with 20g of olive oil. Gently tip the cold dough into the pan. Do not force it to the corners yet. Let it rest and come to room temperature for 2-3 hours until it naturally spreads and looks bubbly.
Step 6: Preheat your oven to 450°F (230°C). Drizzle the remaining olive oil over the dough. Use your fingertips to press straight down into the dough to create deep dimples.
Step 7: Sprinkle with fresh rosemary and flaky sea salt. Bake for 20-25 minutes until the top is a deep golden brown and the edges are crispy. Cool in the pan for 5 minutes before transferring to a wire rack.