The key to a great granita is starting with fresh fruit. If you can find a whole watermelon, start there and scoop out the flesh.
Remember to pre-freeze a small roasting tin to make the freezing process as quick as possible.
As the granita freezes, scraping up the surface every couple of hours creates nice shards instead of a solid block of ice.
When fully frozen, cover the granita and return to the freezer until you’re ready to serve.