Combine 1 tbsp of the crushed peppercorns, the rosemary,mint,garlic,vinegar,soy sauce, and red wine in a large shallow bowl. Add the lamb and turn to coat with the marinade. Marinate the lamb, covered, in the refrigerator, for 8 hours, turning occasionally.
Preheat the oven to 350.
Remove the roast from the marinade and drain; reserve the marinade. Roll the roast, tying it with kitchen twine.
Spread the mustard over the meat and pat 2 tbsp of the crushed peppercorns into the mustard. Set the roast in a shallow roasting pan just large enough to hold it comfortably and pour the reserved marinade carefully around but not over the roast.
Bake for 1 1/2 hours or 18 minutes per pound, basing occasionally. The roast will be medium rare. Bake for another 10 to 15 minutes for well done meat. Let the roast stand for 20 minutes before carving. Serve the pan juices in a gravy boat along with the lamb.