Preheat your oven to 350°F and lightly grease a 9×13-inch baking dish with nonstick spray, setting it aside for later use.
In a large mixing bowl, combine the cream of chicken soup, cream of mushroom soup, sour cream, melted butter, chicken broth, salt, pepper, and garlic powder. Whisk until the mixture is smooth and well blended.
Fold in the cooked spaghetti, shredded chicken, and frozen peas, ensuring everything is evenly coated with the creamy mixture.
Transfer the pasta and chicken mixture into the prepared baking dish, spreading it out evenly. Sprinkle the top with mozzarella, cheddar, and Parmesan cheeses.
Cover the dish with aluminum foil and bake for 30 minutes. Afterward, remove the foil and bake for an additional 15 minutes, or until the cheese is fully melted and the casserole is heated through.
Once done, take the casserole out of the oven and garnish it with freshly chopped parsley. Let it rest for 5-10 minutes before serving.
Enjoy your comforting and creamy chicken spaghetti casserole!