Roast leg of lamb is an elegant dish to serve guests, along with spring vegetables like baby green beans, asparagus or baby summer squash. A semi-boned leg of lamb has had the aitch bone removed. It is part of the hip, an awkwardly-shaped bone attached to the leg by the hip joint. Removing it makes carving the leg of lamb easier. The secret to juicy, flavorful leg of lamb is using an instant-read thermometer to check for doneness. This is the best way to avoid overcooking the lamb.
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Pat the lamb dry on all sides with paper towels. Use the small, sharp knife to remove excess fat and make shallow cuts in the remaining fat. Use the butcher's twine to tie the lamb into a compact roast. Put the lamb in a shallow pan.
Combine the garlic, parsley, rosemary, lemon juice, salt and pepper in a blender and roughly chop. Add the oil in a slow stream while you continue blending the mixture to a paste. Spread it over the lamb and rub it into the shallow cuts in the fat.
Preheat the grill to high. Place the lamb roast on the grill and cook on one side for 10 minutes. Turn the roast and cook on the other side for 10 minutes. Reduce the heat to medium or move the roast to a cooler area of the grill and cover it with foil or close the lid of the barbecue grill. Continue cooking until the internal temperature of the roast is 135 degrees Fahrenheit in the thickest part. Start checking the temperature after an hour of cooking at the lower temperature. Measure the temperature with an instant-read thermometer, and avoid touching the bone when measuring the temperature.
Transfer the roast to a serving platter and allow it to rest for 15 to 20 minutes before slicing.