Add some salt to a pot of water and bring to a boil.

Add the potatoes, and boil 30 minutes or until tender but still a bit firm. Drain, peel, and chop the potatoes.

Put the eggs in a saucepan and cover with cold water.  Bring to a boil.

Cover, and remove from the heat.  Let eggs stand in the hot water for 10 to 12 minutes.  Remove from the hot water and chop.

Combine the onion, celery and egg in a large bowl.

In a separate small bowl, whisk together the salt, pepper, mayonnaise and mustard.  Add this to the potato mixture and toss to coat.

Chill in the refrigerator before serving.