In a large bowl, combine the warm water, instant yeast, and olive oil. Let it sit for about 5 minutes until slightly foamy.
Add the bread flour and salt, then stir until a sticky dough forms. Avoid adding extra flour, the dough should be soft and slightly wet.
Cover the bowl and let the dough rise for 1-2 hours, or until doubled in size.
Generously oil a baking pan. Transfer the dough to the pan and gently stretch it toward the edges. If it resists, let it rest for 10 minutes before stretching again.
Cover and let the dough rise for another 30-45 minutes until puffy.
Preheat your oven to 425°F (220°C). Drizzle the dough with olive oil, then press deep dimples all over the surface with your fingertips. Sprinkle with rosemary and flaky sea salt.
Bake for 20-25 minutes, or until the focaccia is deeply golden with crisp edges.
Let cool for about 10 minutes before slicing and serving.