Preheat your oven to 350°F. Lightly grease an 11 x 14-inch baking dish with cooking spray to prevent sticking and ensure easy serving.
In a large mixing bowl, combine the frozen hash browns, shredded cheddar cheese, sour cream, melted butter, cream of chicken soup, and finely chopped onions. Season with salt and black pepper, then stir everything together until well combined and evenly coated.
Transfer the mixture into the prepared baking dish, spreading it out evenly to ensure uniform baking. Place the dish in the preheated oven and bake for approximately 45 minutes, or until the top develops a golden-brown crust and the casserole is bubbling around the edges.
Remove from the oven and let it rest for a few minutes before serving. Enjoy warm as a comforting side dish.