Mix pepper and mustard; rub all over roast. Insert oven-safe meat thermometer lengthwise into the centre of roast so that shaft is not visible. Place on rack in shallow roasting pan.
Cook roast, uncovered, in 350°F (180°C) oven until meat temperature reads (140°F/60°C for medium-rare or 155°F/68°C for medium) about 30-40 minutes. Cooking time is a guideline only. Roasts may be done sooner or later than estimated time. For an estimated cooking time chart, refer to the oven roasting cooking lesson.
Remove roast, cover with foil and let stand 10 to 15 minutes.
Meanwhile add ¾ cup (175 mL) water, milk, brandy and sauce mix to roasting pan. Bring to boil, whisking constantly; reduce heat and simmer 1 minute. Serve sauce over thinly sliced tenderloin.