In a large mixing bowl, combine the lukewarm water and active dry yeast. Let it sit for about 10 to 15 minutes until the yeast becomes foamy and activated.

In a separate bowl, whisk together the flour and kosher salt. Gradually pour the yeast mixture into the flour mixture, stirring with a rubber spatula or wooden spoon until a sticky dough forms. Do not overmix.

Drizzle a small amount of olive oil over the dough. Cover the bowl tightly with plastic wrap or a clean kitchen towel and let the dough rise at room temperature for 1½ to 2 hours, or until doubled in size.

Grease a 9×13-inch baking pan generously with butter, olive oil, or cooking spray.

Transfer the risen dough into the prepared pan. Using lightly oiled fingertips, gently stretch the dough toward the edges of the pan. If the dough pulls back, let it rest for a few minutes and continue stretching again.

Cover loosely and allow the dough to rise for another 45 minutes until puffy.

Preheat the oven to 425°F. Position the oven rack in the center.

Brush 1 tablespoon olive oil evenly over the top of the dough. Lightly oil your hands, then press your fingertips firmly into the dough to create deep focaccia dimples.

Sprinkle generously with flaky sea salt and fresh rosemary leaves if using.

Bake for 25 to 30 minutes, or until the top is golden brown and lightly crisp.

Remove the focaccia from the oven and carefully transfer it to a cooling rack. Let it cool for at least 10 minutes before slicing.

For sandwiches, allow the focaccia to cool completely before cutting.