01 - Place watermelon cubes in a blender. Add lime juice and sugar. Blend until completely smooth, ensuring no chunks remain.
02 - Taste the mixture and adjust sweetness by adding more sugar if desired. If using mint, stir in the chopped leaves until evenly distributed.
03 - Pour the mixture into a shallow, freezer-safe baking dish. Spread evenly to ensure uniform freezing. Place in freezer for approximately 90 minutes.
04 - Remove from freezer. Using a fork, scrape the entire surface firmly to break up forming ice crystals. Return to freezer immediately.
05 - Continue freezing and scraping every 30 minutes for 3 to 4 hours total. The mixture should become light, fluffy, and grainy in texture.
06 - Spoon the granita into chilled glasses or bowls. Garnish with fresh mint leaves or lime zest if desired. Serve immediately while still icy.