Stir the soup, milk and black pepper with a whisk or fork in a small bowl. Layer half the potatoes, half the onion, and half of the soup mixture in a (1 1/2 qrt) casserole dish. Repeat the layers. Place the butter over the soup mixture. Sprinkle with the paprika. Cover.
Bake at 400 degrees F for 1 hour. Uncover and bake for 15 mins. more or until the potatoes are fork tender.