Preheat the oven to 220°C (425˚F). Brush the top and sides with the mustard. Sprinkle with the pepper, thyme and salt. Set the roast in a roasting pan.
Roast 20 min. at 220°C (425˚F), then reduce oven temperature to 160°C (325˚F). Use an instant-read thermometer to check for an internal temperature of of 50°C (120˚F) for rare, 55°C (130°F) for medium-rare and 71°C (160°F) for medium. As an example, a 1.75 kg/3.5 lb roast requires the 20 min. at 220°C (425˚F) then about another 1 hr. cooking at 160°C (325˚F) to reach medium-rare.
After roast reaches temperature, transfer it to a platter, loosely tent with foil to rest 15 min. Thinly slice to serve. If you would like jus, stir the beef stock into the drippings in the roasting pan, simmer on the stovetop 5 min. Pour into gravy boat to serve alongside roast.