Preheat your oven to 350°F and grease your 9x13-inch baking dish really well with butter.
Make sure hash browns are totally thawed, then press them with paper towels to get moisture out.
Mix hash browns, cream of chicken soup, melted butter, and diced onion together in your big bowl.
Toss in 2 cups of shredded cheese and black pepper, then stir everything until it's all combined.
Spread your mixture into the baking dish and press down gently so it's nice and even everywhere.
Put the remaining cheese on top and bake without covering for 45 minutes until it's golden and bubbly.
Take it out and wait 5 minutes before cutting so the casserole can set up properly.
Add some fresh parsley if you want, and serve it warm with your favorite breakfast foods. YUM!