Prepare Jello according to directions, mix in fruit, and let set in refrigerator. While the jello is setting prepare the topping: In a small sauce pan combine juice, flour, butter, sugar and salt together and let thicken over low heat making sure the mixture in completely smooth. Remove from heat and let cool completely. Mix thickened mixture with cool whip and spread over jello. Refrigerate and Chill Before serving sprinkle topping with chopped nuts or shredded cheese as desired.