Cook macaroni according to package directions.
While that cooks, melt butter in large skillet.
Add onion and cook 3-5 minutes until onion is soft.
Whisk in flour, making a smooth paste.
Gradually whisk in milk until it's all combined and smooth.
Cook over medium heat until it comes to a low boil.
Remove from heat and stir in cheese.
Add salt and pepper to taste.
Drain macaroni and return it to the pan it was cooked in.
Pour sauce into macaroni and add tomatoes and green chilies and chicken.
Stir to combine and serve.