01 - Combine watermelon cubes, lime juice, and sugar in a blender. Process until completely smooth, ensuring no fruit chunks remain.
02 - Taste the mixture and add additional sugar if needed. If using mint, fold 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 the dish from the freezer. Using a fork, scrape the surface firmly to break up forming ice crystals throughout the mixture.
05 - Return to the freezer. Every 30 minutes, remove and scrape thoroughly with a fork. Repeat this process for 3 to 4 hours until the granita achieves a light, fluffy, snow-like consistency.
06 - Spoon the granita into chilled glasses or bowls. Garnish with fresh mint leaves or lime zest if desired. Serve immediately while still icy.