In a large bowl, whisk together the flour, kosher salt, and instant yeast. Add the lukewarm water and use a rubber spatula to mix until the liquid is absorbed and the ingredients form a sticky dough ball. Cover the bowl and let rest for 30 minutes. If desired, perform a stretch and fold: fill a small bowl with water, wet your hand, and, working around the edge of the dough, stretch and fold it toward the center 8 to 10 times until the dough becomes smooth and cohesive.

Rub the surface of the dough lightly with olive oil to prevent drying. Cover the bowl with a lid or plastic wrap and refrigerate for at least 12 hours, or up to 3 days. I make sure the dough is well slicked with oil to keep the top from forming a crust.

Line two 8- or 9-inch pie plates or a 9×13-inch pan with parchment paper or grease with butter. Pour olive oil into the center of each pan. Using two forks, deflate the chilled dough by pulling it to the center, then split into two equal pieces (or leave whole if using a 9×13-inch pan). Place each piece into a prepared pan and roll to coat in oil, forming a rough ball. I find that properly greasing the pan ensures an easy release after baking. Rest the dough, uncovered, at room temperature for 3 to 4 hours.

Preheat the oven to 425°F with the rack in the middle. If using rosemary, sprinkle it over the dough. Drizzle olive oil over each dough round. Rub your hands in the oil, then use your fingers to create deep dimples all over, gently stretching the dough to fill the pan if needed. Sprinkle flaky sea salt generously on top. For an extra hit of flavor, I like to use a little more rosemary than called for.

Transfer the pans to the preheated oven and bake for 25 to 30 minutes, until the underside of the focaccia is golden and crisp. Remove from the oven and place on a cooling rack. Let cool for at least 10 minutes before slicing and serving. For the best texture, I let the focaccia cool completely if I plan to slice it for sandwiches.