1. In a large bowl, whisk together the flour, salt, and yeast. Add lukewarm water and mix with a spatula until a sticky dough forms. Coat dough generously with olive oil to prevent drying. Cover with a damp towel or plastic wrap and refrigerate for 12 to 72 hours to allow a long, cold fermentation, which develops flavor and improves texture by breaking down starches and strengthening gluten.
2. Prepare baking pans (glass, ceramic, or metal) by greasing with butter or lining with parchment paper, then drizzle with olive oil.
3. Remove dough from refrigerator and deflate gently with a fork or by hand, dividing if necessary. Transfer dough to prepared pans, roll or press it lightly to coat with oil, and stretch it to fit. Let rise uncovered at room temperature for 3 to 4 hours until puffed and nearly doubled. For a quicker version, allow dough to rise at room temperature for 1.5 to 2 hours until doubled, then proceed with second rise for about 30 minutes.
4. Preheat oven to 425°F (220°C). If using rosemary, sprinkle it evenly on top. Drizzle remaining olive oil over the dough, then create dimples by pressing gently with oiled fingers to promote characteristic focaccia texture. Sprinkle flaky sea salt evenly across the surface.
5. Bake on the middle oven rack for 25 to 30 minutes until the focaccia is golden brown and crisp on the edges and bottom. Remove from pan and cool on a rack for about 10 minutes before slicing.
6. Store cooled focaccia in airtight containers at room temperature for up to 3 days or freeze for up to 3 months. To refresh crust, reheat in the oven at 350°F (175°C) for 15 minutes.