Mix the Dough: In a large mixing bowl, whisk together the all-purpose flour, kosher salt, and instant yeast. Pour in the warm water and use a silicone spatula to mix until a sticky, shaggy dough forms and no dry spots of flour remain. There is no need to knead it.

First Rise (Overnight): Drizzle 1 tablespoon of extra virgin olive oil over the sticky dough and use your hands or a spatula to turn the dough so it is completely coated in the oil. Cover the bowl tightly with plastic wrap and place it in the refrigerator to rest and ferment for at least 12 hours, or up to 24 hours.

Prep the Pan: The next day, pour 3 tablespoons of extra virgin olive oil into a standard 9x13 inch metal baking pan. Use your fingers to spread the oil evenly across the bottom and up the sides of the pan.

Second Rise: Remove the dough from the refrigerator. Gently gently deflate it and transfer it into the oiled baking pan. Turn the dough over once so it is coated in oil. Use your fingers to gently stretch it out toward the edges of the pan. If it shrinks back, let it rest for 15 minutes and try again. Cover the pan loosely with a clean kitchen towel and let it rise in a warm spot for 2 to 4 hours, or until it is very puffy and fills the pan completely.

Preheat and Dimple: Preheat your oven to 425°F (220°C). Pour the remaining 2 tablespoons of olive oil evenly over the top of the puffy dough. Rub a little oil on your fingertips, then press your fingers straight down into the dough, creating deep dimples all over the surface. You should feel the bottom of the pan with your fingers.

Add Toppings and Bake: Sprinkle the fresh rosemary leaves and the flaky sea salt evenly over the dimpled dough. Bake on the middle rack of the preheated oven for 25 to 30 minutes, or until the top is deep golden brown and the edges are crispy.

Cool and Serve: Remove the pan from the oven. Carefully use a spatula to transfer the hot focaccia out of the pan and onto a wire cooling rack so the bottom does not get soggy. Let it cool for at least 20 minutes before slicing and transferring to your serving dish.