Preheat the oven and position the rack in the center to ensure even baking.
Allow the frozen hashbrowns to thaw completely, then drain off any excess liquid so the casserole does not turn watery.
Combine the hashbrowns, part of the shredded cheese, cream of chicken soup, sour cream, onion, melted butter, garlic, salt, and pepper in a large bowl, mixing until everything is evenly incorporated.
Transfer the mixture into a lightly greased baking dish and spread it out into an even layer.
Sprinkle the remaining shredded cheese evenly over the top to create a golden, bubbly crust.
Bake until the casserole is heated through and the top is lightly browned with bubbling edges.
Remove from the oven, garnish with parsley if desired, and allow it to rest briefly before serving.