1. Prepare Your Oven and Baking Dish

• Preheat your oven to 350°F (175°C).
• Lightly grease a 9×13-inch baking dish with butter or non-stick spray and set aside.

2. Sauté the Onion

• In a large skillet, melt 3 tablespoons of butter over medium heat.
• Add the finely chopped onion and sauté for 3-4 minutes until softened and translucent. Stir occasionally to avoid browning.

3. Make the Roux

• Sprinkle the flour over the onions and stir to coat.
• Cook for 1-2 minutes, stirring constantly, to eliminate the raw flour taste.

4. Add Milk and Chicken Stock

• Gradually whisk in the milk and chicken stock, ensuring no lumps remain.
• Whisk continuously until the mixture thickens, about 3-5 minutes.

5. Add Cheese and Seasonings

• Remove the skillet from heat and stir in the salt, pepper, shredded sharp white cheddar, and Monterey Jack cheese.
• Stir until the cheese melts completely, creating a smooth, creamy sauce.

6. Combine Hashbrowns and Sauce

• In a large mixing bowl, combine the thawed hash browns with sour cream.
• Pour the cheese sauce over the hashbrowns and mix until fully incorporated.

7. Assemble the Casserole

• Transfer the hashbrown mixture into the prepared baking dish and spread it evenly with a spatula.

8. Prepare the Crunchy Topping

• In a small bowl, combine the lightly crushed Cornflakes with melted butter.
• Spread the Cornflake mixture evenly over the top of the casserole.

9. Bake the Casserole

• Place the casserole in the preheated oven and bake for 40-45 minutes, or until the top is golden brown and the edges are bubbling.

10. Optional Extra Cheese

• For extra cheesiness, sprinkle additional shredded cheese on top during the last 5 minutes of baking.

11. Rest and Garnish

• Let the casserole rest for 5-10 minutes before serving.
• Garnish with freshly chopped chives for a pop of color and added flavor.