Preheat the oven to 350°F (180ºC) and grease a 9x13 inch baking dish.
In a large bowl, combine the diced chicken, cream of chicken soup, sour cream, milk, frozen mixed vegetables, garlic salt, and black pepper. Spread the chicken mixture evenly in the prepared baking dish.
In a medium saucepan, bring 1-½ cups of water to a boil. Stir in the butter until melted, then remove from heat and stir in the stuffing mix until well combined. Top the chicken mixture in the casserole dish with the prepared stuffing.
Bake in the preheated oven for 45 minutes, until the stuffing is golden brown and the casserole is bubbling.
Notes
To store: Allow it to cool before transferring it to an airtight container. Refrigerate promptly and enjoy within 3-4 days or freeze for up to 2-3 months. I recommend freezing individual portions, but if freezing the whole casserole, freeze it with a layer of plastic wrap and foil.To reheat: If frozen, thaw overnight in the refrigerator. To reheat individual servings, use the microwave, heating on medium power until thoroughly heated. For the whole casserole, reheat in the oven at 350°F (180°C). Cover it with aluminum foil to prevent over-browning and reheat for 10-20 minutes.