In a large bowl, stir together the warm water, sugar, and instant yeast. Add the flour, fine sea salt, and 2 tablespoons (30 ml) of the olive oil. Stir with a sturdy spoon until a shaggy, sticky dough forms.

Turn the dough onto a lightly floured surface and knead for about 5 minutes, adding only a little flour if needed to keep it manageable. The dough should remain soft and slightly tacky.

Pour 1 tablespoon (15 ml) olive oil into a clean large bowl. Place the dough in the bowl, turn it to coat, cover, and let rise in a warm spot until doubled in size, about 1 hour to 1 hour 15 minutes.

Coat a 9-by-13-inch rimmed baking pan with 2 tablespoons (30 ml) olive oil. Transfer the risen dough to the pan and gently stretch it toward the corners. If it springs back, rest it for 5 minutes and stretch again. Cover and let rise until puffy, about 25 minutes.

Meanwhile, heat the oven to 425°F (220°C). Drizzle the remaining 1 tablespoon (15 ml) olive oil over the dough. Oil your fingertips, then press firmly all over the surface to make deep dimples. Sprinkle with rosemary and flaky sea salt.

Bake until the focaccia is deeply golden on top and bottom, about 20 to 24 minutes. Carefully lift it from the pan and cool on a wire rack for at least 10 minutes before cutting into squares.