In a large mixing bowl, combine the cooked rice, shredded chicken, mixed vegetables, cheddar cheese, cream of mushroom soup, milk, salt, and black pepper.
Transfer the mixture to a baking dish and spread it out evenly.
Bake in the preheated oven for 45 minutes, or until the top is golden and bubbly.
Let it cool for a few minutes before serving.
Notes:
This casserole is perfect for using up leftovers. Feel free to substitute ingredients based on what you have on hand.