Heat oven to 350°F. Spray 12x8-inch (2-quart) baking dish with nonstick cooking spray. In large skillet, brown ground beef and onion until beef is thoroughly cooked. Drain. Stir in tomato sauce, garlic powder and pepper. Spoon into sprayed baking dish. Spoon corn evenly over meat mixture.
Bring water to a boil in medium saucepan. Remove from heat; stir in milk and potato flakes with fork. Spread potato mixture evenly over corn; sprinkle with cheese. Top with green chiles.
Bake at 350°F. for 30 to 35 minutes or until bubbly around edges and cheese is melted. Let stand 5 minutes. Top with chopped tomato.