Method : Bring a kettle of water to a boil and keep hot. Using a very sharp knife, make a deep slit on the leg of lamb along the bone. Working sideways, debone the leg. Manipulate the meat so that you end up with 1 or 2 large pieces which are no more than about 3 inches thick and may be laid more or less flat. Warm vegetable oil over medium heat in a very large, heavy pot with a tight-fitting heavy lid. When the oil is very hot, add the whole onion and the lamb bone. Cook, turning the onion and the bone from time to time, until the onion is deep brown and the bone is pink (but not necessarily in that order). Remove the onion and the bone and discard. Add chopped onions to the oil and let simmer over medium heat until translucent, about 3 minutes. Add carrots and cook, stirring, until the mixture has become orange and the onions are deep golden, about 8 minutes. Add garlic cloves and cook for another 3-5 minutes. Add approximately 2 cups of boiling water. Stir in cumin, coriander, barberry seeds, and black pepper. Cover, reduce the flame, and cook for 5 minutes. Pour the rice on top. Even out the top, but do not mix. Using a chopstick or a skewer, make several deep holes in the rice. Pour enough hot water (getting some into the holes), until the liquid is merely level with the rice. Cover and reduce the flame to very low. Season the meat on both sides with salt and pepper. Sear on a grill or in a skillet with a small amount of vegetable oil, about 4 minutes per side. Place the meat on top of the rice in a way that as much of the rice is covered as possible. Replace the lid and cook over low heat for 45 minutes. DO NOT REMOVE THE LID DURING COOKING! At the end of the 45 minutes, remove the lamb and set aside. Using a spoon, ruffle the top of the rice to mix the grains, but take care not to mix with the ingredients on the bottom. Even out the top, replace the lamb over the rice, cover, and cook for another 45 minutes. Remove the meat again. Mix the rice with the vegetables thoroughly. Adjust the salt, if necessary. Even out the top, replace the lamb, cover, and cook for another 15 minutes. Remove the meat to a cutting board and slice into large portion-sized pieces. Turn the rice out onto a large platter in a mound. Arrange the meat on top. The pilaf may be additionally decorated with roasted heads of garlic.