Preheat your oven to 350°F (175°C).
In a large mixing bowl, combine the frozen hashbrowns, sliced kielbasa, diced onion, shredded cheddar cheese, sour cream, cream of chicken soup, garlic powder, black pepper, and salt. Mix well until all ingredients are evenly distributed.
Pour the mixture into the 9x13 inch baking dish. Spread it out evenly with a spoon or spatula.
Cover the dish with aluminum foil (if desired, to help retain moisture) and place it in the preheated oven.
Bake for 35 minutes covered, then remove the foil and bake for an additional 10 minutes, or until the casserole is hot and bubbly, and the top is lightly golden.
Once done, remove from the oven and let it cool for a few minutes before serving.
Garnish with chopped green onions if you like.