To prepare the salad, boil the water in a medium-size saucepan. Empty the gelatin packets into a large bowl and pour in the boiling water. Mix until dissolved and then stir in the cold water. Chill until the Jell-O just starts to thicken. (You don't want it to be set.) Fold in the apples, celery, and nuts then pour the mixture into gelatin mold or a 12'' x 8'' pan. Chill until firm.
To prepare the topping, combine the sour cream and mayonnaise in a small bowl. Fold in the marshmallows. When the Jell-O is firm, spread the topping on top. (Or, if using a gelatin mold, release the Jell-O onto the platter and fill the center with the marshmallow topping.) Chill for 2 hours before serving.
QVC is not responsible for the availability, content, security, policies, or practices of the above referenced third-party linked sites, nor liable for statements, claims, opinions, or representations contained therein. QVC's Privacy Statement does not apply to these third-party web sites.