Grease an 11" cast iron skillet with 1 tablespoon olive oil.
Combine 3 tablespoons of the olive oil with the flour, water, instant yeast and salt in the bowl of an electric mixer. With the paddle or hook attachment, beat at high speed until well combined and sticky, about 1 minute.
Transfer the batter to the prepared cast iron skillet and spread it out with your fingers to reach the edges of the skillet. Cover it and let it rise at room temperature until doubled in size, about 1 hour.
When the dough has risen, preheat the oven to 375°F. Gently poke the dough all over with your index finger.
Drizzle it with the remaining 2 tablespoons olive oil, and sprinkle with the flaky sea salt and fresh rosemary.
Bake the bread until it's golden brown, 25 to 30 minutes. Allow the bread to cool for 5 minutes, then cut as desired.
Notes
Tip: Don't skimp on the olive oil. The olive oil helps the focaccia brown on top. You don't want to drown the bread, either. When in doubt, use a pastry brush and brush on oil instead of drizzling.