01 - In a large mixing bowl, whisk together the flour, salt, sugar, and instant yeast until evenly distributed.
02 - Pour in the lukewarm water and olive oil. Stir with a wooden spoon or your hands until a sticky, shaggy dough comes together.
03 - Turn the dough out onto a lightly floured surface. Knead firmly for 8 to 10 minutes until the dough becomes smooth, elastic, and springs back when pressed.
04 - Shape the dough into a ball and place it in a lightly oiled bowl. Cover with a damp kitchen towel or plastic wrap. Let it rest in a warm, draft-free spot for 1 to 1½ hours, or until doubled in volume.
05 - Generously coat a rimmed baking sheet (approximately 12x16 inches) with olive oil. Gently transfer the risen dough onto the tray and stretch it out to fill the pan. Press your fingertips firmly into the surface to create characteristic dimples across the entire dough.
06 - Drizzle the remaining olive oil evenly over the dough. Scatter the rosemary leaves across the surface and finish with a generous pinch of flaky sea salt.
07 - Cover the tray loosely and let the dough rise for another 30 minutes until puffy. Meanwhile, preheat your oven to 425°F.
08 - Place the tray in the center of the oven and bake for 20 to 25 minutes until the top is deeply golden and the bottom sounds hollow when tapped.
09 - Remove from the oven and allow the focaccia to cool slightly on the tray. Slice into squares or rectangles and serve warm.