1. Preheat oven to 325°F (160° C).
2. Mix together the pepper and mustard. Brush mixture on all sides of the roast.
3. Place the beef in a small roasting pan or metal baking dish and roast until desired temperature is reached (140°F/60°C for rare or 160°F/70°C for medium) about 20 to 25 minutes per 1 lb/500 g.
4. Remove the roast from the oven and tent with foil.
5. Stir the water, milk, brandy and sauce mix into the juices in the roasting pan. Bring to boil over medium-high heat, whisking constantly.
6. Reduce the heat and simmer 1 minute.
7. Serve brandy gravy over sliced roast beef.