Combine the first six ingredients in a small bowl. Cover and store the sauce in the fridge until needed. Preheat the oven to 450 degrees F. Place the prime rib bone side down in a roasting pan; season with salt and pepper. Roast for 20 minutes, and then reduce oven temperature to 325 degrees F. Finish cooking the roast to the desired doneness, allowing 20 minutes per pound (total cooking time) for rare; 25 minutes for medium rare; and 30 minutes for well done. When the beef is ready, transfer to a plate, loosely cover, and allow it to rest for 10 minutes before slicing. Serve the horseradish sauce alongside.