Combine bread flour, warm water, instant yeast, and fine sea salt in a large bowl. Mix until no dry flour remains. The dough will be very wet and sticky. Do not add more flour. Cover and rest 30 minutes.
Wet your hands. Grab one side of the dough, stretch it up as far as it will go without tearing, and fold it over the center. Rotate the bowl a quarter turn and repeat for four folds total. This is one set. Cover and rest 30 minutes.
Repeat the stretch and fold process every 30 minutes for a total of 3 sets, or up to 4 if the dough still feels slack. With each set the dough will become smoother, stronger, and more elastic.
After the final set, cover and let the dough bulk rise at room temperature until doubled in size and visibly bubbly, approximately 1 to 1.5 hours. Watch the dough, not the clock. A warmer kitchen will speed this up significantly.
Pour 60 grams of olive oil into a 9x13 pan and spread it around the bottom and up the sides. Turn the dough directly out of the bowl into the oiled pan. Let it rest in the pan for 15 minutes so the gluten relaxes. If your pan is not nonstick, line with parchment.
After the rest, wet your hands and gently spread the dough out toward the edges of the pan, handling it gently to keep the bubbles intact. It will not reach the corners perfectly and will continue to spread as it proofs.
Cover and let rise about 1 hour, until light, fluffy, and jiggly when you gently shake the pan.
Preheat oven to 430 degrees F. Drizzle 2 to 3 tablespoons olive oil over the top. Run hands under cool water and shake off the excess so hands are just barely damp. Starting at the edge closest to you, press fingers straight down to the bottom of the pan and work across in rows to create dimples. Sprinkle with flaky sea salt and rosemary if using.
Bake 25 to 30 minutes, checking at 25, until deep golden on top and crisped on the bottom. Rest in pan 5 minutes, then transfer immediately to a wire rack.