In a large bowl, stir together lemon juice, sugar and water, until the sugar has dissolved.
Transfer the granita mixture into a large metal container (a loaf tin works great), cover with cling film, and place in the freezer for half an hour.
Use a wooden spoon to stir vigorously the mixture and break any ice crystals, then transfer the tin back in the freezer.
Repeat every 30 minutes for 2-3 times, or until your granita has reached a velvety icy texture. Alternatively, you can also use an immersion blender to break the ice crystals. Once it reaches the desired consistency, serve straight away.
Store your granita in an air-tight container in the freezer for up to 1 day. Let it defrost for about 10 minutes at room temperature, then give it a nice mix and serve.