Buying and trimming: When ordering the rib roast from a butcher, be sure to request a "top choice" roast cut from the small loin end; the best being ribs 12 through 10. Have the butcher cut off the chine/backbone. The rib bones look best if they are shortened and frenched, have the butcher do this for you as well.
Put the onions, carrots, parsnips, and garlic into the bottom of a large roasting pan. Place the roast, rib side down, on top of the vegetables. In a small bowl mash together the horseradish, garlic, rosemary, thyme, salt, pepper, and olive oil to make a paste. Smear the paste generously over the entire roast and drizzle with some extra oil. Roast for about 1 1/2 to 2 hours or approximately 20 minutes per pound for medium-rare. Check the internal temperature of the roast in several places with an instant-read thermometer, it should register 125 degrees F. for medium rare. Remove the beef to a carving board, cover it with foil, and allow it to rest for 20 minutes. Note: the internal temperature of the meat will continue to rise about 10 degrees. Remove the vegetables and set aside. Pour the pan juices into a fat separator and set aside to allow the fat and beef juices to separate. Use the juices to serve with the meat and save the fat for Yorkshire pudding.