1. Melt butter or margarine over low heat in a medium-size saucepan; stir in flour and salt; cook, stirring all the time, just until mixture bubbles.
2. Stir in milk; continue cooking and stirring until sauce thickens and bubbles 3 minutes; remove from heat. Blend in mayonnaise or salad dressing.
3. Cut a thin slice across top of each potato, then scoop out potato in big chunks; dice and add to the sauce. Fold in the ham, celery and onions.
4. Heap mixture into potato shells; garnish each with a few carrot sticks.