Have butcher prepare lamb by trimming skin and removing most of fat from leg.
Place leg in roasting pan and pour barbecue sauce over meat. Marinate 1 to 2 hours in refrigerator.
Preheat oven to 375 Â°F (190 Â°C).
Transfer roasting pan with lamb from refrigerator to oven. Cook 45 minutes, basting frequently. Season with salt and pepper.
Preheat barbecue at medium.
Oil grill and set leg on grill. Cook 20 minutes, uncovered. Baste lamb 7 to 8 times during cooking and turn leg over 2 to 3 times. Season to taste.
Serve with sweet potatoes.