In a small bowl, combine the crabmeat, cheese, onion and butter.
Sprinkle steaks with salt and pepper. Cut a horizontal slit through each steak to within 1/2 in. of the opposite side, forming a pocket. Fill with 1/2 cup crab mixture. Secure with kitchen string if necessary.
Broil 4 in. from the heat for 7-9 minutes on each side or until meat reaches desired doneness (for medium-rare, a thermometer should read 135°; medium, 140°; medium-well, 145°). Let stand for 5 minutes before serving.