In a medium mixing bowl, DISSOLVE gelatin and salt in boiling water; STIR in cold water, vinegar, and onion; CHILL gelatin until consistency of unbeaten egg white.

STIR cabbage, carrots, celery and green pepper into thickened gelatin mixture.

POUR into a lightly oiled mold (see note); CHILL until firm.

UNMOLD salad mixture onto individual lettuce leaves, TOPPING each serving with teaspoon of mayo.