The steak should be roughly 1 1/2" thick. Place in a shallow dish. Heat the oil separately. Cut the lemon into wedges. Add the garlic and lemon and saute until the lemon peel is lightly browned. Pierce the meat at intervals with a fork. Pour the hot oil over the meat. Place lemon and garlic pieces around the steak. Turn the meat in the marinade. Cover and marinate at room temperature 1 or 2 hours, turning the meat two or three times. Remove the steak - save the marinade. Place the steak on a broiler rack, 4 to 5 inches from the heat. Broil until browned (about 5 minutes per side). Blend the soy sauce, brown sugar and mustard. Spread half the paste on the steak and broil one minute. Turn the steak. Spread the remaining paste on the other side of the steak and broil 1 minute. This produces a rare steak - best for this dish. Carve across the grain in thin slices. This steak also works very well as a left over for sandwiches (try it on Rye Bread).