Preheat your oven to 350°F and lightly grease a 9x13-inch casserole dish.
In a large mixing bowl, combine the thawed hash browns, melted butter, condensed cream of chicken soup, sour cream, diced onion, 1 ½ cups of shredded cheese, and black pepper. Stir everything together until evenly blended.
Transfer the hash brown mixture into the prepared baking dish, spreading it out smoothly. Sprinkle the remaining ½ cup of shredded cheese over the top.
Bake in the preheated oven for 45 to 55 minutes, or until the casserole is hot, bubbly, and the cheese on top is perfectly melted and slightly golden.
Let it rest for a few minutes before serving, and enjoy!