In a large bowl, stir together the warm water, yeast, and sugar. Let it sit for 5 to 7 minutes until it is foamy on top. This tells you the yeast is alive and active.

Add 3 tablespoons of the olive oil and the kosher salt to the yeast mixture and stir to combine. Add the flour all at once and stir with a wooden spoon until a sticky, shaggy dough forms. It will be too wet to knead, and that is exactly right. Do not add more flour.

Cover the bowl with plastic wrap and let the dough rise in a warm spot for 1 hour, until it has doubled and is full of bubbles. If your kitchen is cool, set the bowl in a slightly warm oven with just the light on.

Pour 2 tablespoons of olive oil onto your rimmed sheet pan and use your hands or a pastry brush to coat the bottom and sides generously. Turn the risen dough out onto the oiled pan. Use your fingertips to gently stretch and press the dough toward the edges of the pan. It may spring back a little. That is fine. Just work slowly and let it relax.

Cover the pan loosely with plastic wrap and let the dough rest for 20 minutes. This short rest is important. It lets the gluten relax so the focaccia bakes up light and airy rather than tight and dense. Preheat your oven to 425 degrees now.

Remove the plastic wrap. Drizzle the remaining olive oil over the surface of the dough. Then press all ten of your fingertips firmly into the dough, moving across the whole surface to create deep dimples. You should feel them go almost all the way through. Scatter the chopped rosemary evenly across the top and finish with the flaky salt.

Bake for 20 to 23 minutes, until the top is golden and the edges are deeply browned and pulling away from the sides of the pan. The bottom should be crisp and golden as well. Slide a spatula under a corner to check.

Let the focaccia cool in the pan for 5 minutes, then transfer to a cutting board and cut into squares or strips. Serve warm. The olive oil in the pan will have crisped the bottom crust beautifully, so make sure every piece gets a bit of that edge.