How to Make It
In a medium skillet, combine the allspice, cinnamon, peppercorns and cloves and cook over moderate heat until lightly toasted, about 1 minute. Transfer the spices to a plate and let cool, then grind them to a powder in a spice grinder.
In a food processor, combine the soy sauce, onion, garlic, scallions, oil, rum, thyme, salt, nutmeg and ground spices and process to a paste. In a large shallow dish, coat the lamb with the marinade. Cover and refrigerate overnight.
Light a grill or preheat a broiler. Grill the lamb over a moderately high flame or broil for about 10 minutes per side for medium rare, rotating and turning the meat for even cooking. Transfer the lamb to a cutting board and let stand, loosely covered with foil, for 10 minutes. Carve the lamb across the grain and serve hot.