Bring a large pot of water to a boil and add the spaghetti. Cook for 2 minutes less than the package directions state. Drain pasta and add to a large mixing bowl with the chicken.
While pasta is cooking, melt the butter in a medium sauce pan over medium heat. Whisk in the flour and cook for 1 minute.
Whisk the milk into the flour and cook until thickened, whisking constantly, about 3 minutes.
Remove the milk mixture from the heat and whisk in 10 ounces of grated cheddar along with the cream cheese, garlic powder, onion powder, salt, and pepper. Whisk until mixture is smooth and creamy and the cheese has fully melted.
Pour the cheese sauce over the spaghetti and chicken and add the canned tomatoes and chiles.
Stir well to combine and spread mixture into a greased 9x13 baking dish. Sprinkle the remaining cheddar over the top.
Bake at 350 degrees for 30 minutes or until the cheese has melted and the casserole is hot and bubbly.
Sprinkle with parsley and serve immediately.