Juice the oranges with the lemon.
Transfer the juice to a food processor or blender, add the melon and strawberries and process until smooth. Taste for sweetness and stir in the honey if necessary. Chill for at least 1 hour, stirring in half the chopped mint 20 minutes before serving.
Ladle the soup into bowls, top with a dollop of natural yogurt and sprinkle with the remaining chopped mint.