Heat the oven to 350°F.
Cook beef and onion in a 10-inch skillet over medium-high heat until the beef is well browned, stirring often to separate meat. Pour off any fat.
Stir the soup, water, chili powder and corn in the skillet and heat to a boil. Spoon the beef mixture into a 2-quart shallow baking dish.
Stir the corn muffin mix, egg and milk in a medium bowl just until it's combined. (The mixture will be lumpy.) Spoon the corn muffin mixture over the beef mixture.
Bake for 30 minutes or until the corn muffin mixture is golden brown. Sprinkle evenly with the cheese.