Heat oven to 350°F. In 10-inch nonstick skillet, cook beef and onion over medium-high heat 5 to 7 minutes, stirring frequently, until brown; drain. Stir in tomato sauce, brown sugar, Worcestershire sauce and mustard. Cook 2 to 3 minutes, stirring frequently, until boiling; keep warm.
In small bowl, stir corn bread mix, milk, oil and egg just until moistened (batter will be lumpy).
Spoon hot beef mixture into 8-inch square (2-quart) glass baking dish. Sprinkle with 3/4 cup of the cheese. Spoon corn bread batter evenly over top.
Bake 25 to 35 minutes or until toothpick inserted in center of topping comes out clean. Sprinkle remaining 1/4 cup cheese over hot casserole.