In large bowl, mix gelatin and sugar; add boiling water and stir until gelatin is completely dissolved.
With wire whip or rotary beater, blend cranberry cordial, sour cream and sherbet. Let stand until mixture is slightly thickened, about 1 minute; fold in walnuts.
Spoon onto almost set cranberry layer; chill until firm. Garnish with frosted cranberries. Makes 16 servings.