Mix the yeast with the sugar in a small bowl for about 30 seconds, until the yeast becomes liquid. Stir in two-thirds of the water and the olive oil.
In a large bowl stir together the flour and the salt. Pour the yeast mixture into the flour and salt along with some of the remaining water, if needed. Mix with a wooden spoon and bring together to form a soft dough. Add more water if the dough is a bit dry.
Turn the dough out onto a lightly floured surface and knead for ten minutes until smooth and elastic.
Place the dough into a large, lightly oiled bowl. Cover with cling film or a damp cloth and leave in a warm place to rise for about 1½ hours until doubled in size.
Preheat the oven to 220C/400F/Gas 7.
When the dough has risen, knead it again for about five minutes on a clean floured work surface to 'knock it back'.
Shape the dough into two rough circles. Place the circles onto two baking sheets, cover with a clean, damp cloth and allow to prove (rise) until double their original size (about 10-15 minutes).
When the dough has risen, use your fingertips to form dimples in the dough. Brush with the olive oil and sprinkle with the salt and rosemary leaves.
Transfer the baking sheets to the oven and bake for ten minutes.
Reduce the heat to 190C/375F/Gas 5 and bake for a further 25-30 minutes until the loaves are golden and cooked-through. If you like, you can spray the bread with water (using a mister) once or twice during baking so that it 'steams' in the oven.
When baked, turn out onto a wire rack to cool slightly. The focaccia is best eaten the day it's made.