Combine pineapple and marshmallows. Cover and refrigerate overnight.
Put the reserved pineapple juice in a saucepan. Add flour, vinegar, sugar and beaten egg. Stir and heat until thick. Cool in the refrigerator overnight.
Add peanuts to apples. Mix in the pineapple mixture and the cooled sauce, then fold in Cool Whip. Refrigerate.
Notes
Source note: Janet Legg writes, ‘I like to use green Granny Smith and Red Delicious apples.’