Place potatoes in Dutch oven or large saucepan; add enough water to cover. Bring to a boil. Reduce heat; cook 10 to 20 minutes or until potatoes are tender. Drain; rinse with cold water to cool. Drain well. Cool slightly.
Meanwhile, place egg in small saucepan; cover with cold water. Bring to a boil. Reduce heat; simmer about 15 minutes. Immediately drain; run cold water over egg to stop cooking. Peel egg; chop.
Cut potatoes into bite-sized pieces; place in large bowl. Add egg, celery and onion.
In small bowl, combine mayonnaise, sour cream, mustard, salt and pepper; blend well. Add dressing to salad; mix gently to coat. Cover; refrigerate at least 2 hours or until serving time.