Place the sugar and water in a saucepan and stir over low heat until dissolved. Set aside to cool. Juice the watermelon in an electric juicer to get 1 litre of juice. Combine with the sugar syrup and lemon juice and pour into a flat metal container.* Place in the freezer. After 2 hours, whisk with a fork and return to the freezer.** Stir to break up the ice crystals at 1-hour intervals until the granita is light and easy to scoop. Serves 4–6.
* It’s important to freeze the granita in a metal tray because the metal keeps the mixture colder and at the right consistency.
** Breaking up the granita at intervals during the freezing time separates the ice crystals before they are fully set.