In a large mixing bowl, combine the cranberries, grapes, marshmallows, apple, sugar, and salt.
Cover the mixture and refrigerate for a minimum of 4 hours, or preferably overnight, to allow the flavors to meld.
Using a handheld mixer with a whisk attachment, whip the heavy cream until stiff peaks form.
Gently fold the whipped cream and chopped walnuts into the chilled fruit mixture until evenly combined.
Serve the salad chilled.