In a large mixing bowl, combine the all-purpose flour, instant dry yeast, granulated sugar, and fine sea salt. Whisk these dry ingredients together to ensure they are well combined.

Add the warm water (105-115°F / 40-46°C) and 3 tablespoons of olive oil to the dry ingredients. Stir with a rubber spatula or a Danish dough whisk until a shaggy, sticky dough forms and no dry streaks of flour remain. Avoid over-kneading at this stage; simply ensure all ingredients are incorporated.

Cover the bowl tightly with plastic wrap or a damp kitchen towel. Let the dough rise at room temperature for 1.5 to 2 hours, or until it has visibly doubled in size and appears bubbly on the surface.

Generously drizzle 2-3 tablespoons of olive oil into a 9x13 inch baking pan, ensuring the entire bottom and sides are coated.

Gently pour or scrape the risen dough into the prepared baking pan. Using well-oiled hands, carefully stretch and pat the dough to fill the pan. If the dough resists stretching, let it rest for 5-10 minutes to relax the gluten, then try again. Do your best not to deflate the dough too much.

Cover the pan loosely with plastic wrap or a clean kitchen towel and let it undergo its second rise for another 30-45 minutes, or until the dough looks visibly puffy and airy.

While the dough is on its second rise, preheat your oven to 425°F (220°C).

Once the dough has completed its second rise, uncover it. Drizzle an additional 2 tablespoons of olive oil generously over the entire surface of the dough.

Using your fingertips, gently but firmly press dimples all over the dough, pushing down to the bottom of the pan. This creates pockets for the olive oil and gives focaccia its characteristic texture.

Evenly sprinkle the chopped fresh rosemary and flaky sea salt over the dimpled dough.

Place the pan in the preheated oven and bake for 20-25 minutes, or until the focaccia is beautifully golden brown on top and the bottom crust is firm and golden when checked.

Carefully remove the focaccia from the oven. Let it cool in the pan for a few minutes, then transfer it to a wire rack to cool completely before slicing and serving. Cooling on a wire rack helps prevent the bottom from becoming soggy.