In a large bowl, whisk together the flour, salt, and instant yeast. Stir in the sliced cherry tomatoes.
Add the water. 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 or plastic wrap and place in the refrigerator immediately for at least 12 hours or for as long as three days.
Line a 9×13-inch pan with parchment paper or grease with butter or coat with nonstick cooking spray.
Pour 2 tablespoons of olive oil into the prepared pan. Using two forks, deflate the dough by releasing it from the sides of the bowl and pulling it toward the center. Rotate the bowl in quarter turns as you deflate, turning the mass into a rough ball. Place the dough into the prepared pan and roll it in the oil to coat it all over. Let the dough rest for 3 to 4 hours at room temperature. No need to cover for this 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 the remaining 2 tablespoons of olive oil over the dough. Rub your hands lightly in the oil to coat them, then, using all of your fingers, press straight down to create deep dimples. If necessary, gently stretch the dough as you dimple to fill the pan. Sprinkle with flaky sea salt all over.
Transfer the pan to the oven and bake for 25 to 30 minutes, until the underside is golden and crisp. Remove the pan from the oven and transfer the focaccia to a cooling rack. Let it cool for 10 minutes before cutting and serving. Let it cool completely if you plan to slice it for sandwiches.
Storage TipsRoom temperature: Once completely cooled, transfer the focaccia to an airtight container and store it at room temperature for up to three days.Freezing: Freeze for up to three months. Thaw at room temperature and reheat in a 350°F oven for a few minutes to restore its texture.
