Blend fresh watermelon chunks until smooth, then strain to remove pulp, yielding about 2 cups of juice.

In a small bowl, sprinkle the unflavored gelatin over 1 cup of cold water and let it sit for about 5 minutes to bloom.

In a mixing bowl, combine the watermelon juice and sugar (if using). Heat the bloomed gelatin in the microwave for 15-20 seconds until dissolved, then stir it into the watermelon mixture.

Pour the mixture into jello molds or cups and refrigerate for at least 4 hours or until fully set.

Once set, unmold the jello and serve with cubed watermelon and mint leaves as garnish.