Cut the nectarines into eight or so pieces and remove the stones. Place in a saucepan. Over a low-ish heat dissolve the sugar and water in another pan. When dissolved completely, turn the heat up and bring to the boil. Once boiling pour over the chopped fruit and leave to cool.
Blend the fruit and syrup mixture in a food processor or blender until smooth. Add lemon juice to taste - you need a little bit of sharpness. I used the entire lemon. Sieve the mixture into a shallow container. Put the container in the freezer, uncovered.
After half an hour use a fork to break up the ice crystals. Repeat every half an hour to an hour for 3-4 hours. To serve, scrape the granita with a fork into a large bowl for sharing.
Either scatter some passionfruit seeds over the granita or squeeze on a little lime juice and serve with lots of spoons.