Place a few handfuls of watermelon chunks into a food processor (or a powerful blender) and process until smooth. Keep adding chunks until you've processed all of the watermelon.

Strain the mixture through a sieve into a large bowl, and discard the remaining pulp and seeds.

Add the sugar to the watermelon juice and stir until dissolved. Then stir in cranberry juice, lime juice and bitters (if using).

Taste the mixture and add more sugar if required, making sure to stir until dissolved.

Pour into 2 rectangular bread pans, and place uncovered in the freezer.

Freeze the mixture until ice begins to form around edges of pan, which takes about 2 hours.

Scrape the mixture with a metal spoon or fork to break up the ice, then return to freezer.

Continue scraping the mixture every 30 minutes until it has completely crystallized and is granular in appearance. This should take about 3-4 hours total, depending on the depth of your pans.

The granita can be kept in the freezer until ready to serve. Be sure to scrape again to loosen the ice crystals immediately before serving.

Garnish with sugared cranberries and watermelon slices, if desired.

Store leftovers in the freezer for up to one week.