In a small bowl, combine warm water, sugar, and yeast. Let it sit for 5-10 minutes until foamy.
In a large bowl, whisk together flour and salt.
Pour the yeast mixture and 3 tablespoons of olive oil into the flour mixture. Mix until a shaggy dough forms.
Turn the dough out onto a lightly floured surface and knead for 8-10 minutes until smooth and elastic. Alternatively, use a stand mixer with a dough hook for 5-7 minutes.
Lightly grease a clean bowl with olive oil. Place the dough in the bowl, turning to coat. Cover with plastic wrap or a damp towel and let it rise in a warm place for 1-1.5 hours, or until doubled in size.
Grease a 9x13 inch baking pan with olive oil. Gently punch down the dough and press it into the prepared pan, spreading it evenly to the edges.
Cover the pan and let the dough rise for another 30-45 minutes, or until puffy.
Preheat oven to 425°F (220°C).
Drizzle the top of the dough with a little more olive oil. Use your fingertips to create dimples all over the surface of the dough.
Sprinkle with flaky sea salt and dried rosemary (if using).
Bake for 20-25 minutes, or until golden brown and the internal temperature reaches 200°F (93°C).
Let cool in the pan for a few minutes before transferring to a wire rack to cool completely. Slice and serve.
Notes
Focaccia bread is best enjoyed fresh. It can be stored in an airtight container at room temperature for up to 2 days.