1Heat oven to 350°F. Spray 1 1/2-quart glass baking dish with cooking spray. In 2-quart saucepan, heat milk, water and butter to boiling. Stir in potatoes just until moistened; let stand 1 minute. Stir with fork until smooth. Stir in corn.
2Spoon half of potato mixture into dish. Sprinkle with 1/2 each of the cheese and onions. Top with remaining potatoes; sprinkle with remaining cheese and onions.
3Bake 10 to 15 minutes or until cheese is melted and onions are golden.