Preheat the oven to 350 degrees F.|
Coat a 9- by 13-inch baking dish with nonstick cooking spray.
In a large skillet, brown the ground beef with the onion over medium heat; drain if necessary.
Add the chili starter and diced tomatoes; cover and cook for 10 minutes, stirring occasionally.
Meanwhile, cook the macaroni according to the package directions; drain and place in a large bowl.
Add the chili, green chilies, and 3 cups cheese; mix well and spoon into the baking dish. Top with the remaining 1 cup cheese, then bake for 12 to 15 minutes, or until heated through and the cheese is melted.