Preheat your oven to 350°F (175°C).
Cook the spaghetti according to package instructions until al dente, then drain.
In a large mixing bowl, combine shredded chicken, cooked spaghetti, cheddar cheese, sour cream, cream of chicken soup, milk, chopped onion, garlic powder, salt, and pepper.
Transfer the mixture into a greased 9x13 inch baking dish.
Sprinkle with additional cheese if desired.
Bake uncovered for 30-35 minutes or until the top is golden brown and the casserole is bubbly.
Let the casserole cool for about 5-10 minutes before serving.
