Preheat your oven to 350°F (175°C).
In a large mixing bowl, combine the cooked spaghetti, shredded chicken, cream of chicken soup, sour cream, garlic powder, onion powder, salt, and pepper.
Fold in the frozen mixed vegetables until evenly distributed throughout the mixture.
Transfer the mixture into a greased 9x13 inch baking dish and spread it evenly.
Sprinkle the shredded cheddar cheese evenly over the top of the casserole, followed by the grated Parmesan cheese.
Cover the dish with aluminum foil and bake for 25 minutes.
Remove the foil and bake for an additional 15 minutes, or until the cheese is bubbly and slightly golden.
Let it cool for 5-10 minutes before serving.