Step 1: In a large bowl, whisk together the active sourdough starter and warm water until mostly dissolved.
Step 2: Add the bread flour and sea salt to the water mixture, stirring with a spatula until no dry streaks of flour remain.
Step 3: Cover the bowl and let the dough rest for 30 minutes for the autolyse phase.
Step 4: Perform a set of stretch and folds by pulling the edge of the dough up and over itself, rotating the bowl 90 degrees each time for a total of four folds. Repeat this every 30 minutes for a total of 2 hours.
Step 5: Cover the bowl tightly and place it in the refrigerator for 24 to 48 hours to undergo a cold ferment.
Step 6: Generously coat a 9x13 inch metal baking pan with 2 tablespoons of olive oil. Gently transfer the cold dough into the pan, flipping it once to coat in oil.
Step 7: Let the dough proof in a warm spot for 2 to 4 hours until it has filled the pan and looks very bubbly.
Step 8: Preheat your oven to 425°F (220°C). Drizzle the remaining olive oil over the top of the dough.
Step 9: Using oiled fingertips, press straight down into the dough to create deep dimples all over the surface.
Step 10: Sprinkle with flaky sea salt and fresh rosemary. Bake for 20-25 minutes until the top is golden brown and the bottom is crispy.
Step 11: Remove from the pan immediately and let cool on a wire rack to maintain the crispy crust.