Place the roast on a flat surface and season all sides with salt and pepper. Combine the mustard and green peppercorns and the lemon juice and rub all over the beef. Place it on a roasting rack. Place the potatoes in the bottom of the roasting pan and season them with salt and pepper. Place the rack in the pan directly over the potatoes.
Place the pan in the center of the oven and cook, undisturbed for 10 minutes. Lower the temperature of the oven to 350 degrees F. Count about 10 minutes for each pound then begin testing the internal temperature. The thermometer should read about 130 degrees F for medium rare. Remember that "carry over" cooking (when the roast comes out of the oven and rests) will mean the temperature will increase a few degrees after the roast is removed from the oven. If it's undercooked, you can always cook it more, but if overcooked, there is no fixing it!
Remove the roast beef and place it on a flat surface for carving. Transfer the potatoes to a serving platter. Serve the roast, whole on the platter to carve at the table. Alternatively, arrange slices of beef over the potatoes and coat with a layer of the pan sauce.