In a large mixing bowl, combine the flour, fine salt, and instant yeast, whisking thoroughly to integrate. Add the lukewarm water and stir using a fork or spatula until a loose, sticky dough forms.

Scrape down the sides of the bowl, then drizzle 2 tablespoons of olive oil around and over the dough, coating it evenly. Cover and allow to rise at room temperature for 1½ to 2 hours until it doubles in size, or refrigerate for 24–72 hours to enhance flavor and texture.

Generously oil a 10×14-inch metal baking pan with 2–3 tablespoons olive oil, ensuring the bottom and sides are coated. Alternatively, use butter or a mixture of butter and oil if using glass or ceramic bakeware.

With a greased hand, loosen the dough from the bowl edges. Perform six stretch-and-folds by lifting the dough at the edge, folding it over itself, and rotating the bowl 90 degrees after each fold to form a round.

Transfer the dough to the prepared pan, allowing it to stretch and rise uncovered until it fills the pan and doubles in volume—approximately 45 minutes. For enhanced texture and flavor, allow it to rest for 2–4 hours at room temperature.

Preheat the oven to 425°F (or 400°F with convection).

Drizzle the remaining tablespoon of olive oil over the dough. Using oiled fingertips, press deep dimples into the surface, gently pushing the dough toward the edges. If the dough is resistant, pause for five minutes before continuing. Scatter the flaky sea salt and rosemary evenly on top.

Place on the middle oven rack and bake for 25–30 minutes until the focaccia is golden, puffed, and the underside is crisp. Let rest for 10 minutes before slicing.