Preheat oven to 400 degrees. Coat a 9 X 9-inch square baking dish with cooking spray.
In a large nonstick pan, cook beef and onion over medium-high heat 5 to 7 minutes, stirring frequently, until brown; drain. Stir in tomato sauce, brown sugar, ketchup, Worcestershire sauce, and mustard. Cook 2-3 minutes, stirring frequently until heated through. If the sauce is too thick, add a little water and mix again.
In a small bowl, stir corn muffin mix, milk, and beaten egg just until moistened (batter will be lumpy).
Spoon hot beef mixture into baking dish. Spoon cornbread batter over top. Use a rubber spatula to spread it evenly. Sprinkle the top evenly with shredded cheese.
Bake, uncovered, for about 25 minutes or until a toothpick inserted in center of topping comes out clean.