Preheat oven to 450 degrees F. Set the beef in a roasting pan, brush with the mustard, and sprinkle with thyme, salt and pepper. Roast 20 minutes, then reduce heat to 325 degrees F. Continue cooking to desired doneness, about 60 minutes more for medium rare, and 75 minutes for medium.
When the roast is 20 minutes from being done, gently boil the potatoes until they are very tender, but still hold their shape.
When the roast is done, set on a plate, tent with foil, and rest 10 minutes. Set the roasting pan on the stovetop over medium-high heat. Add the stock/flour mixture and bring to a simmer. Simmer until thickened gravy forms, then reserve on low heat.
When the potatoes are tender, drain well and thoroughly mash. Beat in the milk, butter, salt and pepper. Slice the beef and arrange on a platter. Serve with the gravy and potatoes in vessels alongside.