Rinse the roast and pat dry. Bring roast to room temperature. Stud prime rib with garlic cloves by cutting slits into the meat and placing cloves inside. Rub Italian dressing mix all over roast. Pour black peppercorns into a shallow dish and dredge the roast in the pepper so that it's completely covered. Transfer to a roasting pan lined in aluminum foil.
Place roast in the oven and cook for 1 hour. Turn oven off and let meat sit in oven until meat reaches the desired degree of doneness, about 130 to 135 degrees F for medium rare. Let rest for 10 minutes before carving.
*Cook's Note: Have butcher remove ribs from roast and tie it.