In a bowl blend together, the soup, milk, salt and pepper. Set bowl aside.
Preheat oven to 375º F.
Grease the inside of a 1-1/2 quart casserole baking dish with butter.
In the baking dish, alternate layers of potatoes, onion, and the soup/milk mixture.
Top the last layer with thin slices of butter, and sprinkle with paprika.
Cover baking dish, and bake in oven for 1 hour.
Uncover baking dish, and continue to bake for 15 minutes more.
Serve immediately, but be careful, as potatoes are 'hot'.
*Note: If desired, you can substitute either a can of cheddar cheese soup, or fiesta nacho cheese soup, for any of the cream of celery, chicken, or mushroom soup.
Additional note: I have also cooked these potatoes in my 1,100 watt microwave oven, which I cooked for 10 minutes on power level 7, then stirred. Cooked for another 10 minutes on power level 7, then stirred. Cooked for 5 more minutes on power level 7, then served.