1. Preheat your oven to 350°F (175°C) and lightly grease a 9×13-inch baking dish.
2. In a large bowl, combine the thawed hashbrowns, sliced kielbasa, shredded cheddar cheese, sour cream, condensed cream of chicken soup, chopped onion, melted butter, salt, and pepper. Mix until all ingredients are well incorporated.
3. Transfer the mixture to the prepared baking dish, spreading it out evenly.
4. Bake in the preheated oven for 45-50 minutes, or until the casserole is bubbly and the top is golden brown.
5. Remove from the oven and let it cool for a few minutes before serving. Garnish with chopped fresh parsley, if desired.
