Preheat Oven: Preheat the oven to 350°F and grease a 9×13-inch baking dish with nonstick spray. Spread the thawed hash browns evenly in the pan and set aside.
Prepare Cheese Sauce: In a large saucepan over medium-low heat, combine milk, heavy cream, dry mustard, salt, garlic powder, parsley, and black pepper. Add grated cheddar cheese, stirring frequently until melted.
Combine and Bake: Pour the cheese mixture over the potatoes in the baking dish. Bake uncovered for about 60 minutes, or until the edges are golden brown. Serve and enjoy!