In a saucepan over medium heat, combine the rhubarb, sugar, and 2 tablespoons water. Bring to a boil, and cook for about 15 minutes, or until rhubarb is mushy. Mix well to form a sauce. Taste, and adjust sugar if needed. Reserve 1 cup. Save any extra for other uses.
In a medium bowl, stir together the raspberry flavored gelatin and 1 cup of the boiling hot rhubarb mixture until completely dissolved. Stir in 1 cup cold water, then mix in the apples and walnuts. Pour into a serving bowl, and refrigerate for 2 to 4 hours, or until set. Garnish with whipped topping or creamy salad dressing before serving if you like.