In a small bowl, mash together the roasted garlic cloves, rosemary, lemon and oil; set aside. With a sharp knife, trim away the white skin that covers the lamb (also called fell) as well as the excess fat. Pull up the layer of fat and fell, cutting as you go to expose the meat underneath. Next, remove the tailbone and hipbone at the large end of the lamb.
Cut off the tailbone that protrudes out of the leg. Then, cut under and around the hipbone, scraping the meat away to expose the leg joint. Cut through the tendons around the joint and separate the hipbone. Cut down the length of the leg and remove the thighbone. The leg of lamb is now boneless. Divide the meat into 3 large pieces, this will facilitate quicker cooking. Score the surface of the lamb and rub it with the garlic paste. Roll up each boneless piece of lamb and tie with heavy kitchen string or butcher's twine to hold the roasts together. Season with salt and pepper. Cover and chill overnight.
Prepare a barbecue. Season the lamb roast generously with salt and pepper, then place on the hot grill. Sear the outside of the meat until brown all around, but do not char. Close the grill cover and roast for 30 minutes. The lamb is done when it is still pink and the internal temperature reads 135 degrees F. Allow the roast to stand 10 minutes to let the juices settle before cutting off the twine and slicing.