Tip the chickpea flour, salt and rosemary into a large bowl and slowly whisk in the water until you have a loose, lump-free batter. Rest the batter for at least an hour and up to 12.
Preheat the oven to 220 celsius.
Take a large flat tin or oven-proof frying pan and heat it up in the oven or on the hob.
Skim off any froth from the top of the batter and then stir in most of the olive oil.
Add the remaining oil to the hot pan, swirling it to create a non-stick surface. Now tip in the batter to a depth of about 1cm/just under 1⁄2 and place in the oven.
Bake for 15 to 20 minutes or until the surface of the farinata is crisp and bubbling.
Give it a few turns of the pepper mill, slice up with a pizza cutter and serve right away.
