Boil water in a pot and in the meantime, squeeze the lemon and filter the juice. When the water boils, add the sugar and cook, stirring for around 10 minutes.
Turn off the heat and leave to cool. Add the lemon juice, stir well, and then pour into a hermetically sealed container.
Leave in the freezer for 12 hours. Break the ice and blend in a mixer for 2 minutes, until it reaches the perfect granita consistency.
Place in the freezer for another couple of hours and then blend again. Put the granita in the freezer for another hour. When it’s time to serve, pour the granita into individual cups and decorate with a bit of lime zest.