Preheat oven to 350°F (175°C).
In a large bowl, combine shredded chicken, cooked pasta or rice, sour cream, cream of chicken soup, cheese, and frozen vegetables. Stir until well mixed.
Season with garlic powder, onion powder, salt, and pepper. Stir again and adjust seasoning to taste.
If using breadcrumbs, mix with melted butter and sprinkle evenly over the casserole.
Transfer mixture to a greased 9x13-inch baking dish. Bake for 25–30 minutes until golden and bubbly.
Let cool for 5 minutes before serving.