Heat oven to 400°F. Generously spray 13x9-inch (3-quart) glass baking dish with cooking spray. In 10-inch skillet, cook beef and onion over medium heat, stirring occasionally, until beef is browned; drain. Stir in soup, vegetables and milk.
In baking dish, stir baking mix and water until moistened; spread evenly. Spread beef mixture over batter. Sprinkle with cheese.
Bake 30 minutes.