Preheat oven to 350°F (175°C) and grease a 9x13 inch baking dish with cooking spray or butter.
In a large mixing bowl, combine the cooked sausage, thawed hashbrowns, cream of chicken soup, shredded cheddar cheese, chopped onion, milk, and salt and pepper.
Mix everything together really well until evenly distributed and coated.
Pour the mixture into the prepared baking dish and spread evenly.
Drizzle the melted butter evenly over the top of the mixture.
Bake for 45-60 minutes in the preheated oven until the center is heated through, edges are bubbly, and top is golden brown. Optionally broil briefly for extra crispiness.
Let it sit for a few minutes before serving.