Mix together sourdough, water, salt, and flour until a sticky dough forms. Let the dough rest for 30min to hydrate.
Stretch and fold the dough. Take the dough from the sides and stretch it up and over the center of the dough. Rotate and repeat, stretching and folding about 6 times. Coat the dough in olive oil, cover, and let the dough rise overnight for 8-12hrs.
Generously coat a baking dish with olive oil. Turn the mixing bowl upside down and let gravity do its magic and let the dough fall into the dish. Scrape any leftover dough out of the mixing bowl.
Fold the four sides of the dough over itself (like closing a cardboard box). Flip it so the seam side is down and let rise uncovered for 4-6 hours.
Preheat the oven to 425F.
Top the dough with corn, mozzarella, and scallions. Drizzle mayo all over (I use regular mayo from a squeeze bottle). Oil your fingers and dimple the dough. Generously sprinkle with black pepper and flakey sea salt.
Bake for 25 minutes or until golden. Let the focaccia cool for 15 minutes in the cast iron skillet before removing it from the skillet and to a cooling rack.