Instructions:
1. Preheat your oven to 375°F (190°C). Grease a 9×13-inch baking dish.
2. In a large bowl, combine the thawed hashbrown potatoes, cooked chicken, cream of chicken soup, sour cream, milk, shredded cheddar cheese, grated Parmesan cheese, melted butter, garlic powder, onion powder, salt, and pepper. Mix well until all the ingredients are evenly combined.
3. Pour the mixture into the greased baking dish and spread it out evenly.
4. Bake in the preheated oven for about 40-45 minutes or until the casserole is hot and bubbly, and the top is golden brown.
5. Remove from the oven and let it cool for a few minutes. Garnish with chopped green onions and cooked bacon bits, if desired.
6. Serve the hashbrown chicken casserole as a main dish with a side of vegetables or a fresh salad.