In a large mixing bowl add your flour, salt, yeast, water, and a sticky ball forms and rub with oil and make sure if thoroughly slathered with oil. Cover with a damp dish cloth and place in the fridge overnight.
The next day, line a 9 by 13 inch pan with parchment paper or grease with butter. I suggest using parchment if you are unsure about how non-stick your pan is. Despite focaccia having a lot of oil it can surprisingly stick to the pan.
Deflate your dough lifting up the edges and form into a ball by tucking in the edge of the dough.
Place a tablespoon of oil into your prepared pan and place your dough ball in the middle of the pan.
Cover with a kitchen towel and let the dough ball rest for 3-4 hours.
Preheat your oven to 425 degrees.
If using rosemary, sprinkle rosemary over your dough, which should have spread out more to fill the pan, and drizzle a tablespoon of olive oil over the dough.
Rinse your finger tips with olive oil and dimple the dough by pressing your fingers into it. Sprinkle with flaky sea salt and place in the oven for 25-30 minutes.
The tops should be golden and the underside should be golden and crispy.
Let cool for at least 10 minutes before cutting and serving.