Pour boiling water over gelatin in a bowl and stir until the gelatin is dissolved.
Gradually add gelatin mixture to sour cream, stirring until well blended.
Chill until mixture is slightly thickened.
Stir in pineapple, carrots, and walnuts.
Turn into a 2-quart mold and chill until firm.
Unmold onto a bed of salad greens on a chilled serving plate.
Garnish as desired.