Dust a large flat baking tray with flour.
Put the flour into a large bowl, 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 250ml/9fl oz 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 five 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 ¾cm/¼in). 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 tray. Flatten it out to about 30cm/12in long and 20cm/8in wide. Cover the dough loosely with oiled clingfilm, making sure it is airtight.
Preheat the oven to 200C/400F/Gas 6.
Leave the dough in a warm place for about an hour, or until it has almost doubled in size. With a floured index finger press holes in the dough at regular intervals, about 4cm/1½in apart in rows across the dough, pressing right down to the bottom. Cut 3cm/1¼in 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 25–30 minutes, or until the bread is well risen, pale golden-brown and feels hollow when tapped underneath.
Remove from the oven, drizzle with the remaining olive oil and leave to cool on the baking tray.