How to Make It
Preheat the oven to 350°. In a large skillet, melt the 1 1/2 sticks of cubed butter. Add the onion and a generous pinch of salt and cook over moderate heat, stirring occasionally, until softened, about 7 minutes. Whisk in the flour and cook, whisking constantly, until bubbling, about 2 minutes. Gradually whisk in the stock and bring to a boil over moderately high heat, then simmer over moderately low heat until the sauce is thickened and no floury taste remains, 8 to 10 minutes.
In a medium bowl, toss the cornflakes with the breadcrumbs and the 1 stick of melted butter. Season with salt and pepper.
In a large bowl, combine the sauce with the potatoes, sour cream, 2 teaspoons of salt and 1 teaspoon of pepper and mix well. Scrape into a 9-by-13-inch baking dish and bake for about 20 minutes, until bubbling. Sprinkle the cornflake mixture evenly over the potatoes and bake for 20 to 25 minutes longer, until the topping is browned and crisp. Let stand for 10 minutes before serving.