In a large mixing bowl, combine the flour, fine salt, and yeast with a whisk. Gently pour in the lukewarm water and mix using a spatula or fork until the flour is fully absorbed and a soft, loose dough forms. Scrape down the sides to tidy, then drizzle 2 tablespoons of olive oil over and around the dough, coating the surface and edges. Cover and allow the dough to rise at room temperature for 1½–2 hours, or refrigerate for 24–72 hours for enhanced flavor and an airy crumb. Pour 2–3 tablespoons of olive oil into the bottom and sides of a 10×14-inch Detroit-style pan, or butter a 9×13-inch baking dish if preferred (refer to notes for ceramic or glass). With oiled hands, loosen the dough from the bowl’s edges and perform six sets of stretch-and-folds, lifting the dough at the edge and folding it over itself, turning the bowl a quarter turn each time to form a smooth ball. Transfer the dough ball into the prepared pan, leaving it uncovered to rise again until doubled and spread across the pan—approximately 45 minutes, or 2–4 hours for optimal texture and taste. Preheat the oven to 425°F (or 400°F for convection). Drizzle the final tablespoon of olive oil over the surface, then press oiled fingertips into the dough to create deep dimples, gently coaxing the dough to the pan’s edges. Should the dough resist, allow it to rest for five minutes before continuing. Sprinkle generously with flaky sea salt and scatter the rosemary over the top. Bake on the middle rack for 25–30 minutes until the focaccia is puffed, deeply golden, and the bottom is crisp. Rest for 10 minutes prior to slicing.