Warm your water to 90℉ to 100℉ and add your yeast and mix. Set aside and let set for 5 to 10 minutes for the yeast to get bubbly and activate. If after 5 minutes there are no bubbles, your yeast is likely no longer active. Start over with new yeast.
While the yeast is activating, in a large bowl, whisk together the flour and salt. Then after 5 to 10 minutes, add the water and yeast mixture.. 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. Add the dough to a bowl and 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. It is important the dough really be covered with olive oil or else it might stick to your cover. If you are using a towel, secure it with a rubber band to make a more airtight cover. If you do not slick the dough with enough oil and do not create a close to airtight cover you risk the dough drying out and forming a crust over the top layer.
If you do not have 12 hours, you can place the bowl on the counter or in a warm space for 3 to 4 hours and then move forward with baking.
Line a 9 × 13 inch pan (see notes above) with parchment paper, or a silicone baking make, or grease with vegan butter or coat with nonstick cooking spray.
Pour 2 tbsp of olive oil into the center of your 9 × 13 inch pan. Using two forks, or your hands, 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.
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.
Set a rack in the middle of the oven and preheat it to 425°F. Pour 2 tbsp of oil over your 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 the rosemary and flakey sea salt over the dough.
Place the pan in 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. If you are halving it to make a sandwich, let the bread cool completely before cutting it.
When the Focaccia has completely cooled, transfer it to a bread bag or paper bag and store it at room temperature for up to 3 days. You can freeze it for up to 3 months. To reheat it, place it in the oven at 350ºF for 15 minutes. This will help to revive the crust and make a great eating experience!