Make the marinade: In a food processor puree the onion, garlic, gingerroot and pepper, scraping down the sides with a rubber spatula, add the soy sauce, brown sugar, and the oil, and blend the mixture until it is just combined. Coat the lamb on both sides with the marinade and enclose it in a plastic bag. Let the lamb marinate, chilled, turning it several times if possible, overnight. Alternatively, let the lamb marinate at room temperature, turning it several times, for 3 to 4 hours. Preheat grill. Grill the lamb boned side up on a very clean rack 2 to 3 inches above glowing coals, basting it several times with the marinade, for 5 to 6 minutes on each side, or until it is seared and charred lightly. Transfer the lamb in a shallow roasting pan to a warm oven (170 degrees F to 190 degrees F) for at least 30 minutes and up to 1 hour. (The lamb will continue to cook as it rests in the warm oven; at serving time a meat thermometer should register about 130 degrees F for medium-rare meat.) Cut the lamb into 1/4-inch slices. Arrange the slices on a heated platter, nap them with some of the remaining pan juices.