1. Place potatoes in a large saucepan; add cold water to cover. Cover and bring to a boil; reduce heat and simmer, partially covered, 10 minutes or until tender. Drain, transfer to a large bowl and cool 5 minutes.
2. Whisk together mayonnaise, sour cream, mustard, salt and pepper in a medium bowl. Combine potatoes, celery, eggs, parsley and mayonnaise mixture. Mix gently with a large rubber spatula. Garnish with additional parsley.