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 wet mixture. Stir with a spatula or your hands until no dry flour remains and a shaggy dough forms.
Step 3: Cover the bowl and let the dough rest for 30 minutes to hydrate.
Step 4: Perform four sets of stretch and folds every 30 minutes. To do this, wet your hands, grab one side of the dough, pull it up, and fold it over to the center. Rotate the bowl and repeat 4 times per set.
Step 5: After the final fold, cover the dough and let it sit at room temperature for 2-4 hours until it has increased in volume by about 50% and looks bubbly.
Step 6: Transfer the dough to a well-oiled 9x13 inch baking pan. Gently flip the dough to coat both sides in oil. Cover and place in the refrigerator for 12-24 hours.
Step 7: Remove the pan from the fridge 2-3 hours before baking. Let it proof in a warm spot until the dough has spread to the corners of the pan and is very jiggly.
Step 8: Preheat your oven to 425°F (220°C). Drizzle the top of the dough with more olive oil. Use your fingertips to dimple the dough deeply, creating pockets for the oil.
Step 9: Sprinkle with fresh rosemary and flaky salt. Bake for 20-25 minutes until the top is deep golden brown and the bottom is crispy.
Step 10: Allow the focaccia to cool in the pan for 5 minutes before transferring to a wire rack to maintain the crispy crust.