Chop the cranberries to a course grind in a food processor (or hand grinder). You don't want them smooth, you want them to have some texture but not large chunks. (Freezing the cranberries prior makes them much easier to grind.)
Place chopped cranberries in a large bowl and mix in the sugar, diced apple and marshmallows. Mix well then refrigerate overnight. (This will allow the cranberries to juice, the sugar to dissolve and the marshmallows to soften.)
A few hours before serving, whip the 1 cup of heavy whipping cream (add sugar to taste, whip till med/stiff peaks)then fold into cranberry mixture. Top with chopped nuts and chill till served.