In a large bowl, whisk together flour, salt, and instant yeast. Add the water. Using a rubber spatula, mix until the liquid is absorbed and the ingredients form a sticky dough ball. Rub the surface of the dough lightly with olive oil. Cover the bowl with a damp tea towel, cloth bowl cover, or plastic wrap and place in the refrigerator immediately for at least 12 hours or for as long as three days.

Line a 9x13-inch pan with parchment paper or grease with butter/coat with nonstick cooking spray.

Pour 2 tablespoons of oil in the center of your pan. Deflate the dough by releasing it from the sides of the bowl and pulling it toward the center. Roll the dough ball in the oil to coat it all over, forming a rough ball. Let the dough ball rest for 3 to 4 hours depending on the temperature of your kitchen (Note: no need to cover for this room temperature rise).

Set a rack in the middle of the oven and preheat it to 425°F. If using rosemary, sprinkle it over the dough. Pour 2 tablespoons of oil over dough. Rub your hands lightly in the oil to coat, then, using all of your fingers, press straight down to create deep dimples. If necessary, gently stretch the dough as you dimple to allow the dough to fill the pan. Sprinkle with flaky sea salt all over.

Bake for 25-30 mins, until the underside is golden and crisp. Transfer the focaccia to a cooling rack. Let it cool for 10 minutes.