1-Preheat oven: Preheat oven to 350 degrees Fahrenheit.

2-Melt butter and sauté onion: Melt 4 tablespoons butter in a large saucepan over medium heat. Add onion and sauté until softened, about 5 to 6 minutes.

3-Add flour and milk: Add flour and cook, stirring constantly, for 1 minute. While whisking, pour in milk and cook, stirring constantly, until mixture comes to a boil.

4-Stir in cheeses: Remove from heat and stir in cheddar and parmesan cheeses until melted.

5-Add sour cream and season: Stir in sour cream, then season with salt and pepper to taste, making sure it’s a bit strong to flavor the potatoes well.

6-Coat hash browns: Place thawed hash browns in a large mixing bowl and pour the cheese mixture over them. Toss well to coat evenly. Transfer mixture to an ungreased 13 by 9-inch baking dish and spread into an even layer.

7-Crush cornflakes: Place cornflakes in a resealable plastic bag and crush into pebble-sized bits using a rolling pin.

8-Coat cornflakes with butter: Add 3 tablespoons melted butter to cornflakes, seal bag, and toss to coat evenly.

9-Sprinkle topping: Sprinkle the buttered cornflake mixture evenly over casserole.

10-Bake casserole: Bake in the preheated oven for 40 to 50 minutes, until hot throughout and topping is crispy.

11-Garnish and serve: Garnish with chopped parsley and serve warm.