The herbs and spices in this marinade enhance the rich, savory flavor of the lamb.
Lay the lamb out flat and sprinkle with salt and pepper on all sides.
Put the oil in a baking dish large enough to hold the lamb. Add the lamb and sprinkle on both sides with mustard seed, cumin, garlic, thyme, fennel seeds, rosemary, bay leaves and lemon juice. Turn and rub the lamb so it is evenly coated with the ingredients. Marinate in a cool place for one or two hours, or up to six hours in a refrigerator. If refrigerated, let the lamb return to room temperature before cooking.
Preheat a charcoal grill or the oven broiler.
Remove lamb from marinade and reserve marinade. If a grill is used, put the lamb flat on the grill. If the broiler is used, place the lamb under the broiler 4 to 5 inches from the heat. Cook the lamb uncovered on the grill or under the broiler about 15 minutes. Turn and cook 10 to 15 minutes on the second side. For medium or well-done meat, cook longer.
Meanwhile, bring the reserved marinade to a boil in a baking pan large enough to hold the lamb, stirring. Remove from heat, and swirl in butter and parsley. Transfer the lamb to the marinade pan, cover loosely with foil and keep warm. Let the meat rest 10 to 15 minutes before serving. Slice thinly and serve with the pan gravy.