Preheat oven to 350°F. Lightly grease a 9×13-inch baking dish with non-stick cooking spray.
In a large mixing bowl, combine the chicken, cream of chicken soup, mayonnaise, milk, diced onion, shredded cheddar cheese, and frozen peas and carrots. Stir well to blend.
Gently fold in the cooked egg noodles, ensuring they are evenly coated.
Transfer the mixture to the prepared baking dish and spread evenly.
Sprinkle the panko bread crumbs evenly over the top.
Drizzle the melted butter evenly over the bread crumbs.
Bake uncovered for 30-35 minutes, or until the casserole is bubbling and the top is golden brown.
Remove from the oven, let sit for a few minutes, then serve warm.