Dissolve gelatin and salt in boiling water.
Stir in cold water and vinegar.
Cool mixture to room temperature.
Blend in mayonnaise and mustard.
Chill till partially set.
Fold in ham, shredded cabbage, and onion.
Pour into 5 or 6 custard cups or individual molds.
Chill till firm.
Unmold on lettuce-lined plates