Hull and halve the strawberries, then place in a large mixing bowl. Stir through the sugar and leave to macerate for 1 hour, or until glossy and juicy. Add the macerated strawberries to a jug, then pick in most of the basil leaves. Squeeze in a little lemon juice and pour in 125ml of cold water.
Blitz to a smooth purée using the multi-purpose blade, then transfer to a large shallow plastic or ceramic dish. Place in the freezer and freeze for 30 minutes, then use a fork to scrape the ice crystals so you have shaved ice. Put the dish back in the freezer and repeat every 30 minutes for a couple of hours, or until you have a completely broken-up granita.
Serve straight away topped with the remaining basil leaves, or if you want to keep it for a later date, make sure you remove it from the freezer around 15 minutes before you want to serve it.