Put the yeast in a mug or small bowl, add the sugar and then enough water to cover the yeast. Mix this with a spoon and set aside for 15 minutes. It will turn foamy after a while.
Put the flour and salt into a large mixing bowl and combine. Add the yeast when it is ready, and the olive oil. Bring all the ingredients together, either with your hands or a table knife, until a dough starts to form. Mix for a minute or so with your hands to really bring it together. Now add a little flour to your work surface and put the dough onto it. It will still be quite sticky at this time. Knead with both hands, stretching, folding and turning the dough repeatedly, adding a little more flour when it sticks too much to the surface. Keep this going for about 10-15 minutes, until you have a smooth elastic dough that doesn’t stick so much to the hands.
Put a little olive oil in a clean mixing bowl and turn the dough around in it to coat it in the oil. Cover the bowl with a clean tea towel and leave in a warm place for about 1 ½ hours, until it has doubled in size.
Grease a large baking tray with olive oil.
Bring the dough back to your floured surface and press and punch all the air out of it. This is the knocking back process, where you are bringing the dough back to the state it was in before it was proven. Place the dough on the baking tray and form a large oval with your hands. Press all of your fingers into the top of the dough to create the indents that you see in focaccia bread, then brush with more olive oil and sprinkle with sea salt and the chopped rosemary. Cover the dough again with the towel and put it back in the warm place for another 20 minutes, until it is about 1 ½ times its size.
Preheat the oven to gas 7/220C/425F.
When the dough is ready, open the oven door and spray water a couple of time into it to create a slightly moist atmosphere. Two sprays will do it. Gently put the bread into the middle of the oven and cook for 25 minutes, until it is risen and browned. Allow to cool on the baking tray for about 10 minutes and then transfer to a cooling rack. Before serving, brush with more olive oil and sea salt if required.