In a large bowl mix water, yeast and sugar. Stir until the yeast is almost dissolved and add 1/4 cup olive oil and salt. Gradually add the flour, mixing with a wooden spoon, until it is absorbed. A sticky and soft dough will form. Cover the bowl with cling film and put it in a warm place for 1,5-2 hours. You can wrap the bowl with tea towels to increase the warmth. The dough should almost double its volume.
Line a big rectangular tin with a sheet of parchment paper. Brush the paper and the sides of the tin with olive oil.
Grease your hands with some of the remaining olive oil. Extend the dough in the prepared tin. Leave the focaccia to in a warm place for another half hour.
Use your fingers to poke the dough in order to form dimples. Sprinkle sea salt and fresh rosemary leaves on top of the focaccia. Optionally brush it with more olive oil.
Bake for 20-30 minutes or until golden brown.
This focaccia is relatively thin. If you want yours to be thicker, bake it in a smaller tin or in a 10-12 inch round cake pan.
Keep leftovers in an airtight container for 2 days. Reheat focaccia in the oven, sprinkled with water and olive oil.