Add water to a small, microwave-safe bowl. Microwave until lukewarm, about 40 seconds.
Add sugar and stir to dissolve. Sprinkle over yeast and stir to combine.
Let sit until frothy, about 15 minutes.
Dough:
Grease a large bowl with 2 tbsp olive oil.
In another large bowl, add flour, salt, and 2 more tbsp olive oil. Pour in yeast mixture. Mix with a wooden spoon until combined.
Knead against the side of the bowl until dough comes together in a loose ball. Transfer to a lightly floured work surface and knead until elastic and smooth, about 5 minutes.
Transfer to a greased bowl and toss to coat dough in oil. Cover with plastic wrap and let rise until twice the size, about 1 hour.
Baking:
Toward the end of the rise, preheat the oven to 400° F.
Grease a 9×13 baking sheet evenly with 2 more tbsp olive oil.
Lightly punch down the dough and transfer to a prepared baking sheet.
Press with your fingers to stretch and guide dough to the edges of the sheet. Make indentations all over as you go.
Drizzle top with 2 more tbsp olive oil and brush to coat. Sprinkle all over with rosemary, flaky salt, and garlic. For an even crunchier golden top, feel free to use more olive oil.
Bake until focaccia is puffy and golden, about 25 minutes.
