Combine onions, soysauce, water, sherry, sugar, sesame seeds, garlic and pepper in a bowl; mis well to dissolve sugar. Place meat in a shallow glass dish and pour marinade over. Prick all over with a fork to allow marinade to penetrate. Cover and marinate in the refrigerator overnight; turn occasionally.
Drain meat, broil or grill to desired doneness, basting occaionally with marinade.