To butterfly a leg of lamb means to bone it so the meat may be spread out in one large piece so you can roast it. It cooks in half the time of a leg of lamb with a bone and makes the carving easy. To prepare the leg of lamb, cut off as much membrane as you can and it is ready to bone. I think the fat should stay as it will melt while it is being roasted. A whole leg of lamb contains the hipbone and the shank bone from the knee to the ankle at the small end. All the boning takes place on the underside of the main leg, not on top. Most smaller legs of lamb have the hipbone removed, leaving the shank bone at the smaller end of the leg. Cut the meat from the sides of the bone and under it, and proceed up to the knee joint. Cut around the main leg bone thus exposing the knee joint and you will free the leg and shank bone in one piece. Cut out the white cartilaginous disk that is the kneecap, as well as chunks of interior fat. Lay the meat out, boned surface up, on your work surface, and you will see that it forms two large lobes. For even cooking, slash the lobes in 2-3 places about 1 inch½ /4 cm deep; otherwise the thick pieces of meat will take longer to cook than the rest. Now sprinkle with salt and pepper and spread a filling of your choice leaving a 1 inch border. Roll the lamb into a tight cylinder. Tie roast securely with kitchen twine i ½ inch intervals. Loop the free end of a ball of string round the end of the meat and tie a knot. Without cutting the string, make successive loops at 1½ inchs/4 cm intervals along the meat; tighten each loop by pulling the string as you go. Secure the lamb by bringing the string under the entire length of the meat and knotting the free end. Brush with oil and roast pork in a preheated oven 375°F/190° until an instant-read thermometer inserted into the center of loin reads 145 °F/63°C .