1.) Heat oven to 350°F. Spray 13x9-inch (3-quart) glass baking dish with cooking spray. In 3-quart saucepan, heat milk, water and margarine to boiling. Stir in contents of both pouches of potatoes (from potatoes box) just until moistened; let stand 1 minute. Stir with fork until smooth. Stir in corn.
2.) Spoon half of potato mixture into dish. Sprinkle with ½ each of the cheese and onions. Top with remaining potatoes; sprinkle with remaining cheese and onions.
3.) Bake 10 to 15 minutes or until cheese is melted and onions are golden.
* Try a new cheese, like a Mexican blend, to add extra flavor.