For the dough, combine the flour, yeast and salt with up to 350ml/12fl oz lukewarm water, until you have a soft dough. Knead for 10-15 minutes. Transfer to an oiled bowl, cover with cling film and leave to rise for 30 minutes.
After 30 minutes, place into a lined and slightly warmed baking tray of 37 x 27cm/14 x 10¾in. Drizzle with olive oil and top with fresh rosemary, chopped garlic, sea salt and a little pepper.
Press your fingers into the dough to make dimples and then rest again for another 30 minutes.
Preheat the oven to 240C/450F/Gas 8.
Bake for about 15 minutes, or until evenly golden-brown. Check the focaccia from time to time as domestic ovens can very often colour one side and not the other. Check and move the tray accordingly.
Once cooked, remove from the oven and immediately drizzle some olive oil all over. Leave to cool, then cut into squares.