In a small bowl, combine the ground coriander, salt, and pepper. Blend well and set aside.
Preheat the oven to 350° F. Line the bottom of a roasting pan with foil and spread the onions, garlic, and ½ cup of water .
Place the lamb on the rack, fat-side up. Sprinkle the coriander, salt and pepper on it, gently rubbing them in. Put in oven and roast for about 2 1/2 to 3 hours. Leave it to rest for 15 minutes before serving.