Cut the honeydew or reticulated melon in half diagonally. Scrape out all the seeds with a spoon and cut out the flesh into balls with a sharp-edged round spoon. Likewise, cut out large and small balls from the watermelon pulp.
Gently mix all the fruit in a baking bowl. Sprinkle with juice of one lemon and sherry, then put to cool for half an hour.
Arrange the fruit decoratively on four plates. Rinse the mint, shake dry. Pluck off the leaves and decorate the fruit with them. Serve the fruit salad slightly chilled.