In large skillet, brown ground beef and onion; drain well. Add tomato sauce, garlic powder, 1/4 teaspoons salt and pepper;.
Spoon into greased casserole. Spoon corn evenly over meat mixture.
In medium saucepan, combine water, butter and 1/2 teaspoons salt; bring to a rolling boil. Remove from heat. Stir in milk and potato flakes with fork until potatoes are desired consistency. Stir in green chiles. Spread potato mixture evenly over corn. Sprinkle with cheese.
Bake at 350F for 30-35 minutes or until cheese is melted and bubbly around edges.