Wash lemon well with hot water and peel the rind in a spiral with a peeler. Cut the melon in half, remove the seeds and use a baller to remove balls from the flesh.
Juice one lemon, cassis, grape juice and lemon peel in a punch bowl. Add the melon balls and chill for 20 minutes.
Fill the punch bowl with well chilled sparkling wine, mix carefully and garnish with a few leaves of lemon balm. Serve either in a punch bowl or in wide glasses.