In a medium bowl, combine ham, celery, onion, mayonnaise, and mustard.
Lightly butter the bread slices, if desired.
Spread 6 slices of bread with even portions of ham salad mixture.
Makes 6 sandwiches.
For 24 small luncheon or party sandwiches, cut each sandwich diagonally into quarters.
Please Note: Nutritional values listed may vary from final results depending on actual measurements and brands used to prepare the recipe.