Combine in bowl, strawberry/banana jello and 1 cup boiling water. Stir until jello dissolves. Add sliced frozen strawberries and stir until strawberries break apart. Add 1 can crushed pineapple, well drained, and 1 mashed banana. Mix until blended. Spoon half of jello mixture into an 8-inch square pan. Chill until set. Set other half aside at room temperature. Combine 1 package of cream cheese, 1 cup sour cream, and 1/2 cup chopped walnuts. Mix until well blended. Spread evenly over set jello. Pour remaining jello mixture over cheese mixture. Chill until set. Cut into squares and serve on crisp lettuce leaves.