With a mortar and pestle, crush the peppercorns together. Combine 1 tablespoon of this mixture (reserve the rest) with the rosemary, garlic, vinegar, wine, and soy sauce in a glass dish large enough to hold the lamb when untied. Place the lamb flat in the marinade and marinate overnight (or 2 days) , turning occasionally.
Preheat oven to 350°F
Remove the lamb from marinade and set marinade aside. Roll lamb up and retie with cotton kitchen string.
Spread mustard all over the lamb and pat reserved peppercorns onto the meat, coating it completely.
Coat a roasting pan (with a rack) with non-stick spray and place lamb on rack in pan.
Roast until done, about 18 minutes per pound.
Remove lamb to a cutting board to rest and place roasting pan on stove-top. Heat marinade in roasting pan, stirring to get brown bits from bottom of pan. Serve juices with lamb.
(Can be left butterflied, and grilled on BBQ. Spread mustard on one side and pat peppercorns into mustard.).