Heat the oven to 350°F. Lightly grease a 9×13-inch baking dish and set aside.

Start the Sauce: Melt the butter in a large sauté pan over medium-low heat. Add the diced onion and cook, stirring often, until soft and translucent, about 2–3 minutes. Add the minced garlic and cook for 30 seconds, just until fragrant.

Make the Roux: Sprinkle in the flour and cook, stirring constantly, for 1 minute, allowing it to lightly toast without browning.

Build the Creamy Base: Slowly pour in the milk, stirring constantly, until smooth and slightly thickened. Remove the pan from heat and stir in the sour cream until fully combined and creamy.

Season and Add Cheese: Stir in the salt, pepper, nutmeg, paprika, and shredded cheddar cheese. Mix until the cheese is melted and the sauce is smooth.

Add the Potatoes: Fold in the thawed hash browns until evenly coated in the creamy sauce.

Assemble: Transfer the mixture to the prepared baking dish and spread evenly.

Add the Topping: Sprinkle crushed potato chips or crackers evenly over the top. If using corn flakes, gently crush them in a zip-top bag, toss with the melted butter, then sprinkle over the casserole.

Bake: Cover and bake for 45–50 minutes, until hot, bubbly, and the potatoes are fork-tender.

Golden Finish: Uncover and broil for 1–2 minutes until the topping is deep golden brown. Watch closely to prevent burning.

Serve hot and enjoy!