Preheat the oven to 425 degrees F. Set the roast in a roasting pan. Brush the top and sides with the mustard. Sprinkle with the pepper, salt and thyme. Roast 20 minutes, and then reduce heat to 325 degrees F. Continue roasting to the desired the doneness; about 60 minutes more should yield a medium-rare roast. Remove from the pan, loosely cover with foil, and let rest 10 minutes before thinly slicing. If you would like jus, pour 3 cups of beef stock into the roasting pan and simmer on the stovetop 5 minutes.