Place roast (fat side up) on a rack in an open shallow roasting pan. Insert meat thermometer so tip is in center of thickest part of meat. Combine and mix roast seasoning ingredients and spread over the surface of the roast with a rubber spatula. Roast meat in a 325°F oven for about 30 minutes per pound to desired degree of doneness, using thermometer as a final guide: (140°F-rare; 150°F-medium; 160°F-well). NOTE: Remove roast from oven when thermometer registers 10°F lower than desired doneness and let set for about 15 minutes before carving.
Remove roast from pan and set aside. Remove any large pieces of charred drippings from pan. Add butter and onion to roasting pan and cook over burner on low-medium heat stirring to soften and cook onion about 3 minutes. Add vinegar and scrape bottom of pan to deglaze drippings. Add flour, stirring and cooking for 1-2 minutes. Stir in wine and beef broth. Add mustard, horseradish, sugar, salt and pepper. Stir until sauce comes to a boil and thickens slightly. Remove from heat. Strain sauce through sieve or strainer. Carve beef and serve with sauce.