Spray a 9x13-inch baking dish with non-stick cooking spray.
In a large bowl, combine the drained chicken, cream of chicken soup, mayonnaise, milk, diced onion, shredded cheese, and frozen peas and carrots. Stir the mixture until everything is well combined.
Gently fold in the cooked egg noodles into the mixture.
Pour the mixture into the prepared baking dish. Spread it evenly.
Sprinkle the top with panko bread crumbs and drizzle melted butter over the breadcrumbs.
Bake uncovered for about 30-35 minutes, or until the casserole is bubbly and the top is golden brown.
Notes
Monitor the casserole towards the end of baking to avoid over-browning the panko topping.
You can cover it with foil if it cooks too quickly.