In a large bowl combine the bread flour, water, active sourdough starter, and salt. Mix well until no dry flour remains. The dough will be very wet and sticky. That is correct. Do not add more flour. Cover and let rest for one hour.

Once the dough has built some strength, perform one slap and fold. Pick the dough up, slap it down onto the surface, and fold it over itself. Return it to the bowl with the same side facing down. Cover the bowl.

Leave the covered bowl at room temperature for 8 to 14 hours depending on your kitchen temperature. The dough is ready when it has doubled or tripled in size, is visibly domed, has bubbles across the surface, and pulls away easily from the sides of the bowl when you run a spatula around the edge. It should be very jiggly when you gently shake the bowl. Watch the dough, not the clock.

Pour the olive oil into the 9x13 pan and use your hands to work it evenly across the bottom, into the corners, and up the sides. If your pan is not a reliable nonstick, lay a sheet of parchment in the pan before adding the oil.

Carefully turn the dough directly out of the bowl and into the oiled pan. Using oiled hands, bring the bottom edge of the dough up to about the two thirds mark, then bring the top edge down to the bottom, layering the dough over itself. Flip the dough so the seam side faces down, then rotate it so it sits vertically in the pan rather than horizontally.

Cover the pan and let the dough rest at room temperature for 2 to 4 hours until it is light, fluffy, and very jiggly when you gently shake the pan. It should look soft and alive. Do not rush this step.

Preheat the oven to 430 degrees F. Run your hands under cool water and flick off the excess so your hands are just barely damp, not dripping. Drizzle 2 to 3 tablespoons of olive oil over the top of the dough. Starting at the edge closest to you, press your fingers straight down through the dough all the way to the bottom of the pan, then pull them out and continue working your way across the entire surface in rows. Press firmly and go all the way down.

Sprinkle generously with flaky sea salt. Bake for 25 to 30 minutes until the top is deep golden and the edges are visibly crisped. Lift a corner to check the bottom. It should be a rich, even golden brown.

Let the focaccia rest in the pan for 5 minutes, then transfer to a wire rack to cool completely before cutting. A fully cooled focaccia slices cleanly and holds up to the toppings.

Slice the cooled focaccia horizontally into about five long sections. Split each section through the middle like a hot dog bun so you have a top half and a bottom half. Stack them back together, then make one cut down the center for larger rectangles or two cuts for smaller squares. Pull the top and bottom apart again so each piece is a single layer, crumb side up. Arrange the pieces on a half sheet pan.

Preheat the oven to 425 degrees F. Spread a thin layer of pizza sauce over each piece. Top generously with shredded mozzarella, then scatter the pepperoni over the cheese.

Bake at 425 degrees F for 10 to 15 minutes, until the cheese is melted and the focaccia edges are crisp. For extra color on the cheese, finish under the broiler for a few minutes, watching closely the entire time so it does not burn.

Let the squares rest for a minute or two so the cheese sets slightly, then serve warm.