In a 1 gallon plastic self sealing bag, mix the beef, egg, cracker crumbs, tomato sauce, salt, pepper and dry mustard. Mix by squeezing the bag and set it aside. Cut off root ends of the onions to ease removal of the center, leaving a 3/4 inch thick shell.
Divide meat mixture into four portions and roll them onto balls. Put the balls in the center of four onion halves. Put the onions back together. Wrap each onion in foil and fold down the foil at the top and on the sides to make a packet. Cook over hot coals for 15 to 20 minutes on each side.