Preheat the oven to 450 degrees F. Combine the first 5 ingredients in a small bowl. Place the prime rib bone-side down in a roasting pan. Brush with the horseradish mixture. Roast for 20 minutes; reduce the 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. If desired, make jus to serve with the roast by simmering 3 cups of beef stock in the roasting pan for 5 minutes.
Options: Try the horseradish crust on another type of beef roast, such as top sirloin or top round. Use chopped rosemary in the crust instead of thyme. Serve Yorkshire pudding with the beef.