Preheat oven to 325 degrees F.
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.