01 - Combine watermelon cubes, lime juice, and sugar in a blender. Process until completely smooth and no chunks remain.
02 - Taste the mixture and add additional sugar if needed. If using mint, stir in the chopped leaves until evenly distributed.
03 - Transfer the mixture to a shallow, freezer-safe baking dish. Spread evenly and place in the freezer for 90 minutes.
04 - Remove from freezer and use a fork to scrape the surface thoroughly, breaking up all ice crystals that have formed.
05 - Return to freezer and repeat the scraping process every 30 minutes for 3 to 4 hours, until the mixture becomes light, fluffy, and icy throughout.
06 - Spoon the granita into chilled glasses or bowls. Optional garnish with fresh mint leaves or lime zest before serving.