In a saucepan, cover potatoes and onion with water. Bring to a boil, and then simmer over medium-low heat for 30 minutes or so, until fully softened. Drain.
Mash potatoes and onion with a potato masher to remove chunks.
In a large mixing bowl, combine partially mashed potatoes, cream cheese, salt, pepper, and milk. Whip together on High for 3 minutes.