Prepare the Hashbrowns:
Pat the thawed hashbrowns dry with paper towels to remove excess moisture.
Mix the Ingredients:
Reserve 1 cup of shredded cheddar cheese for topping. In a large bowl, combine the hashbrowns, melted butter, cream of chicken soup, sour cream, pepper, and the cooked onions. Stir in most of the cheddar cheese, leaving the reserved cup for the top.
Bake the Casserole:
Transfer the mixture to a greased 9×13-inch casserole dish, spreading it evenly. Top with the reserved cheddar cheese. Cover with aluminum foil and bake for 45 minutes.
Crisp the Top:
Remove the foil, increase the oven temperature to 400°F (200°C), and bake for an additional 10-15 minutes, or until the top is golden and crispy.
Cool and Serve:
Let the casserole cool for 10 minutes before serving. Enjoy!
