Refrigerate marshmallows and pineapple, covered, overnight.
In a small nonreactive saucepan, stir together reserved pineapple juice, sugar, flour, vinegar and egg. Cook over medium-low heat, stirring, for about 5 minutes or until temperature reaches 160 degree F on an instant-read thermometer. Refrigerate, covered, in a bowl overnight.
In a large bowl, combine the two chilled mixtures with the whipped topping, apples and 1-1/4 cups of the chopped nuts. Sprinkle remaining 1/4 cup nuts on top. Chill until ready to serve. Makes 10 servings.