1. In a small nonstick skillet coated with cooking spray, saute onions in butter until tender; set aside.
2. Meanwhile, in a large bowl, combine the corn, muffin mix, egg whites and milk. Pour into a 9-in. square baking dish coated with cooking spray. Combine sour cream and onions; spread over batter. Sprinkle with cheese.
3. Bake, uncovered, at 350° for 25-30 minutes or until a toothpick inserted near the center comes out clean. Yield: 9 servings.