Preheat your oven to 375°F (190°C). Generously grease a 9x13 inch casserole dish.

Ensure your thawed hashbrowns are very dry. Place them between layers of paper towels or a clean kitchen towel and press firmly to squeeze out as much moisture as possible. This is the key to a non-watery casserole. Set aside.

In a large saucepan or Dutch oven, melt 1/2 cup of butter over medium heat. Add the chopped onion and cook, stirring occasionally, until softened and translucent, about 5-7 minutes.

Stir in the minced garlic and cook for another minute until fragrant.

Sprinkle the all-purpose flour over the onions and cook, stirring constantly with a whisk, for 1 minute to cook out the raw flour taste. This creates a roux.

Gradually whisk in the whole milk until the mixture is smooth. Continue to cook, whisking frequently, until the sauce thickens and begins to bubble, about 3-5 minutes.

Remove the saucepan from the heat. Stir in the sour cream, 1 1/2 cups of the cheddar cheese (reserving 1/2 cup for the topping), all of the Gruyère cheese, salt, pepper, and smoked paprika. Stir until the cheese is fully melted and the sauce is smooth and creamy.

In a large mixing bowl, combine the dry hashbrowns with the prepared cheese sauce. Gently fold together until the hashbrowns are evenly coated.

Pour the mixture into your prepared casserole dish and spread it into an even layer.

In a small bowl, combine the crushed cornflakes and 4 tbsp of melted butter. Toss to coat.

Sprinkle the reserved 1/2 cup of cheddar cheese over the casserole, then evenly distribute the buttered cornflake mixture on top.

Bake uncovered for 45-50 minutes, or until the casserole is hot, bubbly, and the topping is golden brown.

Let the casserole rest for 10-15 minutes before serving. This allows it to set up and makes it easier to serve.