Preheat your oven to 350°F (175°C) and lightly grease a 9x13-inch baking dish with butter or cooking spray.
Ensure your frozen hash browns are completely thawed. Place them in a colander and gently press with paper towels to remove excess moisture for crispier edges.
In a large mixing bowl, combine the sour cream, condensed soup, melted butter, diced onion, garlic powder, salt, and pepper. Whisk until smooth.
Add the thawed hash browns to your creamy mixture, gently folding to ensure even coating without breaking the shreds.
Fold in 1½ cups of the shredded cheese, reserving ½ cup for the topping.
Spread the mixture evenly in your prepared baking dish.
In a small bowl, combine the crushed cornflakes with 2 tablespoons of melted butter, then sprinkle this mixture over the casserole.
Sprinkle the reserved ½ cup of cheese over the cornflake topping.
Bake uncovered for 45-55 minutes, until the top is golden brown and the edges begin to bubble. The internal temperature should reach 165°F.
Allow the casserole to rest for 5-10 minutes before serving. Garnish with fresh parsley if desired.