Preheat oven to 375 degrees. Mix together Salt, Pepper, Garlic Powder and Rosemary. Rub roast all over with dry rub. Place roast on rack in middle of oven. Roast 1 hour. Turn oven off. Do not open oven at any time. May be held in oven for several hours. Turn oven on again to 375 degrees for 30 minutes before serving for rare, 40 minutes for medium rare. Works for any size prime rib roast.
While cooking mix together, Mayo, Sour Cream, Horseradish, and Ground Onion. Chill and serve with dinner.