A simple, no‑knead focaccia made with a high‑hydration dough that ferments in the refrigerator for 12–18 hours (or up to three days). The result is a pillowy, olive‑oil‑crusted loaf that’s ready in about 30 minutes of bake time.
In a large bowl whisk together the flour, kosher salt, and instant yeast. Add the lukewarm water and mix with a rubber spatula until a sticky dough forms. Rub the surface of the dough lightly with olive oil, cover with a damp cloth bowl cover or plastic wrap, and place in the refrigerator for at least 12 hours (up to three days).
Line two 8‑ or 9‑inch pie plates or a 9×13‑inch pan with parchment paper, or grease them with butter (or a thin coat of olive oil).
Remove the dough from the fridge, deflate it and split it into two equal pieces (or keep it whole for a single pan). Lightly coat each dough ball with olive oil and place into the prepared pans. Let the dough rest at room temperature for 3‑4 hours.
Preheat the oven to 425 °F. If using rosemary, sprinkle it over the dough. Drizzle the remaining olive oil (about a tablespoon per pan) over the surface and press down with your fingertips to create deep dimples. Sprinkle flaky sea salt over the top.
Bake the focaccia for 25‑30 minutes, until the underside is golden and crisp. Transfer the pans to a cooling rack and let the bread cool for 10 minutes before cutting.
When completely cooled, store the focaccia in an airtight container at room temperature for up to 3 days, or freeze for up to 3 months. Reheat at 350 °F for 15 minutes before serving.