Dust a large flat baking sheet with flour.
Put the flour in a large bowl and add the salt and yeast, then add the olive oil plus enough warm water to make a soft but not sticky dough. The dough should feel quite loose and not tight and difficult to knead. If the whole amount is added it may appear that the dough is beyond repair, but gently kneading by way of scooping up the dough, scraping any sticky bits on the surface and slapping it back down again for a few minutes, will see the dough begin to become "pillowy" and more manageable. The more water that can be added (the full 1 cup/9fl oz/250ml is great), then the lighter the bread will be. But it can take some perseverance. Also resist the temptation to add more flour, as it will make the dough too heavy.
Knead the dough for about 10 minutes by hand on a lightly floured work surface or for 5 minutes if using an electric mixer fitted with a dough hook. The dough will feel stretchy when pulled. To test if it is ready, make a ball with the dough then, using a well-floured finger, prod a shallow indent in the side (no more than 1/4in/3/4cm). If the indent disappears by way of the dough springing back, then it is ready to shape. If the indent
stays, knead for a few minutes longer. Shape the dough into an oval and place it on the prepared baking sheet.
Flatten it out to about 12in (30cm) long and 8in (20cm) wide. Cover the dough loosely with oiled plastic wrap, making sure it is airtight. Preheat the oven to 400 degrees F (200 degrees C).
Leave the dough in a warm place for about 1 hour, or until it has almost doubled in size. With a floured index finger press holes in the dough at regular intervals, about 1 1/2in (4cm) apart in rows across the dough, pressing right down to the bottom. Take 1 1/4in (3cm) long sprigs of the rosemary and push them into the holes. Sprinkle some sea salt over the dough and place in the top third of the oven. Bake for about 25 to 30 minutes, or until the bread is well risen, light golden brown and feels hollow when tapped underneath.
Remove from the oven, drizzle with the remaining olive oil and let cool on the baking sheet. This is totally awesome when served warm as a starter or indeed as a meal in itself with fresh tomatoes, artichokes, and cold meats, or with a steaming hot bowl of soup.