Combine the flour, yeast, water, rosemary, and salt in a medium-sized bowl. Mix until all the flour is combined. You can use a spoon or a rubber spatula to do this.
Cover with saran wrap and move this to the refrigerator to chill for 12-18 hours.
The next day, remove it from the fridge. It will have risen and doubled.
You can either use 1- 9x13 pan or 2- 9x9 inch pans. Spray the with pan release or brush with butter.
Add in ⅓-1/2 cup of olive oil. I used 2- 9x9 pans and added ⅓ cup of olive oil to each. If you are using 1 pan, add ½ cup of olive oil to the pan.
Either divide your dough in half and add it to each pan or add to 1 pan.
Cover and let them sit at room temperature for 4-6 hours. Keep an eye on it. You want it to be bubbly and light. It will also have spread across and filled in the pan.
Once it is risen and ready, preheat the oven to 425 degrees.
Drizzle the focaccia with another ¼ cup of olive oil. Sprinkle with the flake salt or you can use kosher if you prefer. Dimple the top and move it immediately to the hot oven.
Bake until it is golden brown on the top. Mine took about 17 minutes.
Remove it from the oven and let it cool for about 5-7 minutes and then place it on a cooling rack. You can serve it right away or store it for later!