Step 1

Preheat the oven to 350F. Lightly grease or butter a 9x13 baking dish and set aside.

Step 2

Combine shredded cheeses in a small bowl and toss to combine. Set aside.

Step 3

Place defrosted potatoes in a very large bowl and toss with 4 tablespoons of melted butter and salt and pepper.

Step 4

Stir in sour cream and 2 cups of the combined cheese. Set aside.

Step 5

Melt 4 tablespoons of butter in a large skillet over medium heat. Add onions and cook until translucent, about 4 to 5 minutes. Add garlic and continue cooking, stirring frequently for 1 minute.

Step 6

Sprinkle with flour and continue cooking for another minute, stirring to combine.

Step 7

Slowly whisk in chicken stock until completely combined and smooth. Slowly whisk in milk until smooth and combined. Bring the mixture to a simmer, season with salt and pepper, and continue cooking for three to four minutes until thickened. Remove from heat.

Step 8

Pour cream sauce over the potato mixture and gently fold to combine. Transfer to prepared baking dish. Top with remaining cheese.

Step 9

Combine 4 tablespoons of melted butter and cornflakes in a medium bowl tossing to combine.

Step 10

Spread the topping over the layer of cheese, making sure it's nice and even. Cover with foil and bake for 20 minutes. Remove foil and bake for another 15 to 20 minutes or until edges are bubbling and cornflakes are golden brown. Let rest for 10 minutes before serving.