Bring large pot of water to a boil; add potatoes (do not peel). Simmer 20 minutes or until a knife pierces center easily. Drain under cool water; when cool enough to handle, peel and cube.
In a separate saucepan, bring water to a boil; add eggs, cover and reduce heat to low. Cook 12 minutes, then drain under cool water. When cool enough to handle, peel four of the eggs and dice. Refrigerate remaining egg until ready to serve salad.
In a serving bowl, combine mustard, relish, onion, salad dressing, sugar, celery seed, salt and pepper. Stir in potatoes and diced eggs; best if made a day ahead. To serve, garnish with remaining egg, peeled and quartered.