Preheat the oven to 350°F. Grease a 9x13-inch baking dish.
Spread the thawed hash brown potatoes evenly in the prepared baking dish.
In a large saucepan, combine milk, heavy cream, dry mustard, salt, garlic powder, onion powder, dried parsley, and black pepper. Cook over medium-low heat, stirring frequently.
Once the mixture is warm, add the shredded cheddar cheese and continue stirring until the cheese is completely melted and the sauce is smooth.
Pour the cheese sauce evenly over the potatoes in the baking dish. Bake in the preheated oven for 50-60 minutes, or until the edges are golden brown and bubbly.
Serve warm, garnished with fresh parsley.