Preheat Oven
Preheat your oven to 350°F (175°C). Grease a 9x13-inch baking dish.
Mix Base
In a large mixing bowl, combine the thawed hashbrowns, cream of chicken soup, 1 1/2 cups of cheddar cheese, sour cream, melted butter, onion, garlic powder, salt, and pepper. Stir until well combined.
Assemble Casserole
Spread the mixture evenly into the prepared baking dish.
Add Topping (Optional)
If using, mix crushed cornflakes or Ritz crackers with 2 tablespoons melted butter. Sprinkle evenly over the casserole.
Bake
Bake uncovered for 45–50 minutes, or until hot and bubbly.
Add Final Cheese
Sprinkle remaining 1/2 cup cheddar cheese on top and bake for an additional 5 minutes, until melted.
Serve
Let the casserole sit for 5–10 minutes before serving to allow it to set slightly.