Meanwhile, sauté the celery and onions in a bit of oil or butter (or both) until soft and translucent. Toss with the bread in a large bowl with the spices. Set aside.
In a small saucepan, heat the butter and milk.
Drain and mash the potatoes. Add the to the seasoned bread along with the celery, onions and butter/milk mixture. Mix with a big spoon until everything is well distributed and fairly smooth. Serve hot immediately (it cools off quickly!).
Notes
The bread should be fresh but you could easily make this with warmed, leftover mashed potatoes. Or honestly, by mixing leftover stuffing and potatoes together.