Get Ready: Start by preheating your oven to 350°F (175°C). Lightly grease a 9x13-inch baking dish and set it aside.
Combine Cheeses: In a small bowl, mix your freshly grated sharp cheddar and Monterey Jack cheeses. Keep them ready.
Prep Potatoes: In a large bowl, combine the thawed hash brown potatoes with 4 tablespoons of melted butter, ½ teaspoon salt, and ½ teaspoon black pepper. Stir in the sour cream and 2 cups of your mixed cheeses. Set this delicious potato blend aside.
Make the Creamy Sauce: Melt 4 tablespoons of butter in a large skillet over medium heat. Add the diced onion and cook until it's soft and translucent, about 4-5 minutes. Stir in the minced garlic and cook for another minute until fragrant. Sprinkle in the flour, stirring constantly for one minute. Gradually whisk in the chicken broth until smooth, then slowly add the milk until everything is well combined. Bring the sauce to a gentle simmer, season with ½ teaspoon salt and ½ teaspoon black pepper, and let it thicken for 3-4 minutes. Remove the sauce from the heat.
Assemble the Casserole: Pour the warm, creamy sauce over your potato mixture and gently fold everything together until well coated. Transfer this comforting blend into your prepared baking dish. Sprinkle the remaining cup of cheese evenly over the top.
Add Crunchy Topping: For the perfect crunchy topping, mix 4 tablespoons of melted butter with the crushed cornflakes in a medium bowl. Evenly spread this buttery cornflake mixture over the cheese layer.
Bake to Perfection: Cover the dish with foil and bake for 20 minutes. Then, remove the foil and bake uncovered for an additional 15-20 minutes, or until the edges are bubbly and the cornflakes are golden brown and crisp.
Rest and Serve: Let the casserole rest for 10 minutes before serving. Enjoy your homemade, cheesy goodness!