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 incorporating mint, fold in the chopped leaves until evenly distributed.
03 - Transfer the blended mixture to a shallow freezer-safe dish, spreading it into an even layer. Place in the freezer for 90 minutes.
04 - Remove the dish from the freezer. Using a fork, scrape the surface firmly to break up forming ice crystals and create a grainy texture throughout.
05 - Return the dish to the freezer. Continue removing and scraping with a fork every 30 minutes for 3 to 4 hours total, until the mixture becomes fluffy and granular throughout.
06 - Spoon the granita into chilled dessert glasses or bowls. Optionally garnish with additional fresh mint or lime zest before serving immediately.