Sprinkle gelatin over boiling water. Remove from the heat; stir until gelatin is completely dissolved, about 5 minutes. Stir in marshmallows until melted. Stir in bananas. Drain pineapple, reserving juice. Add pineapple and cold water to gelatin mixture.
Pour into a 13-in. x 9-in. pan; chill until set. In a small saucepan, combine the sugar and flour. Gradually stir in reserved pineapple juice. Add butter; bring to a boil. Cook and stir for 2 minutes. Remove from the heat; cool to room temperature, about 35-40 minutes.
In a small bowl beat the cream until firm peaks form; fold into pineapple juice mixture. Spread over gelatin mixture. Sprinkle with nuts. Chill for 1-2 hours.