How to Make It
In a saucepan, melt the butter. Add the onion and cook over moderate heat, stirring, about 7 minutes. Whisk in the flour and cook for 1 minute. Slowly whisk in the milk. Cook over moderate heat, whisking, until thickened, 5 minutes. Off the heat, whisk in the blue cheese and season with salt and cayenne.
Put half of the potato chips in a large shallow bowl and drizzle with half of the blue cheese sauce. Add the remaining chips and sauce to make another layer. Serve right away.