To make a turkey casserole, peel and cut 6 medium potatoes and place them into a Dutch oven. Cover the potatoes with water and bring them to a boil over high heat. Reduce the heat, cover the Dutch oven and cook for 10 to 15 minutes.
While the potatoes cook, melt 1/2 cup of butter in a large pan. Saute 1 1/4 cups of chopped celery and 3/4 cup of chopped onion until they are tender. Remove the pan from the heat, and set it aside.
In a large bowl, mix 6 cups of unseasoned stuffing cubes, 1 teaspoon of poultry seasoning and 1/4 tablespoon of rubbed sage. Add the onion mixture and 1 cup of chicken broth, and lightly stir. Transfer the stuffing to a greased 13 x 9-inch baking dish.
In a separate large bowl, mix together 4 cups of cubed cooked turkey, 2 cans of undiluted condensed cream of chicken soup, 1 teaspoon of garlic powder and 1/4 cup sour cream. Spoon the turkey mixture over the stuffing.
Drain and mash the potatoes in a large bowl. Beat in 4 ounces of softened cream cheese, 1/2 cup of sour cream, 1/2 teaspoon of pepper and 1/4 teaspoon of salt. Spread the potatoes over the turkey mixture and sprinkle it with 1 1/2 cups of shredded cheddar cheese. Bake the casserole, uncovered, for 30 to 35 minutes at 350 F until the dish is heated through.