Preheat oven to 325 degrees F. Place roast bone-side down in a roasting pan. Brush top and sides of roast with mustard. Sprinkle with thyme, pepper and salt. Cook to desired doneness, allowing 20 minutes per pound for rare, 25 for medium rare, and 30 for well done. When beef is ready, transfer to a plate, loosely cover, and rest 10-15 minutes before thinly slicing. To make jus to serve with the roast, remove fat from pan, pour in three cups of beef stock, and simmer 5-10 minutes.