Heat oven to 400 degrees. Cook ground beef and 1/2 cup of the onion in a 10 inch skillet until beef is brown; drain. Stir in barbecue sauce. Spoon into ungreased 13x9x2 baking dish; sprinkle with cheese.
Stir baking mix, milk and eggs until blended. Pour over beef mixture. Sprinkle with remaining onion. Bake 25-27 minutes or until crust is light golden brown.