In a large mixing bowl, whisk together the flour, instant yeast, and kosher salt until evenly combined.

Pour in the lukewarm water and stir with a sturdy spatula or wooden spoon until no dry flour remains and a shaggy, sticky dough forms.

Drizzle 1 tablespoon olive oil over the surface of the dough and turn it lightly so the top is coated.

Cover the bowl tightly and refrigerate for at least 12 hours and up to 24 hours, with 18 hours being ideal.

Generously oil a 9 x 13-inch metal baking pan with 2 tablespoons olive oil.

Remove the dough from the refrigerator and gently release it from the sides of the bowl with oiled hands or two forks. Transfer it to the prepared pan.

Turn the dough once in the pan so it is lightly coated with oil on both sides, then nudge it into a rough rectangle.

Let the dough rise at room temperature for 2 to 4 hours, until it looks soft, puffy, and nearly fills the pan.

Preheat the oven to 425 degrees Fahrenheit.

Drizzle the remaining 1 tablespoon olive oil over the dough and sprinkle with rosemary, if using.

Lightly oil your fingertips and press straight down all over the dough to create deep dimples. Sprinkle evenly with flaky sea salt.

Bake for 25 to 30 minutes, rotating the pan once if needed, until the top is deeply golden brown and the edges are crisp.

Remove the focaccia from the pan and cool on a wire rack for at least 10 minutes before slicing and serving.