Mix jello with boiling water and dissolve. Stir in 2 tablespoons of sugar and 7-Up. Refrigerate until the mixture is set but still liquid enough to add marshmallows, pineapple (drain juice and save for topping) and bananas. Stir these ingredients together and return to the refrigerator for a firm set.
For the topping, combine 1/2 cup of saved pineapple juice, 1 beaten egg, 1/2 cup sugar, flour and butter in a sauce pan. Bring to a low boil and thicken. Cool slightly to add whipped cream. Mix well then spread generously over firmly set jello.