In a small bowl, combine the ground coriander, salt, and pepper. Blend well and set aside.
Place the oven rack as close to the center as possible, then preheat to 350° F.
Line the bottom of a roasting pan with heavy-duty foil.
Spread the cut onions, garlic, and ½ cup of water on top of the foil. This will impart a lively flavor and fragrance to the recipe during roasting.
Place a roasting rack over the onion mixture.
Place the lamb on the rack, fat-side up. Leaving the fat on will flavor and moisten the meat.
Sprinkle the coriander, salt and pepper on the surface of the lamb, gently rubbing them in.
Roast the lamb for about 2 1/2 to 3 hours, basting periodically with the juices from the bottom of the pan. (There is no need to turn the meat during roasting.)
Remove the roast to a carving board, allowing the meat to rest for about 15 minutes before slicing.
Serve with pan juices that have been skimmed of fat.