Heat oven to 400 degrees. Generously grease 9x13-inch baking dish, or 10 inch pie dish. Cook beef and onion in 10-inch skillet until beef is brown; drain. Stir in soup, vegetables, and milk. Stir baking mix and water in baking dish until moistened, then spread evenly. Spread beef mixture over batter. Sprinkle with cheese. Bake 30 minutes.