Step 1: In a large bowl, whisk together the active sourdough starter, warm water, and honey until the starter is mostly dissolved.
Step 2: Add the bread flour and sea salt to the liquid mixture and stir with a spatula until a shaggy dough forms and no dry flour remains.
Step 3: Cover the bowl and let the dough rest for 30 minutes to autolyse.
Step 4: Perform the first set of stretch and folds by pulling the edge of the dough up and over into the center, rotating the bowl and repeating 4-5 times.
Step 5: Repeat the stretch and folds 3 more times at 30-minute intervals. The dough will become smoother and more elastic.
Step 6: Cover the bowl tightly and let the dough ferment at room temperature for 4-6 hours, then transfer to the fridge for 12-24 hours for a cold ferment.
Step 7: Generously coat a 9×13 inch baking pan with olive oil. Gently transfer the cold dough to the pan, being careful not to pop the bubbles.
Step 8: Let the dough proof in the pan in a warm spot for 2-4 hours until it has spread to the corners and is very jiggly.
Step 9: Preheat your oven to 425°F (220°C). Drizzle more olive oil over the top and use oiled fingertips to dimple the dough deeply.
Step 10: Sprinkle with flaky sea salt and rosemary, then bake for 20-25 minutes until the top is deep golden brown.
Step 11: Remove from the oven and let cool in the pan for 5 minutes before transferring to a wire rack to maintain the crispy bottom.