How to Make It
Preheat the oven to 350°. Butter a shallow 1 1/2-quart gratin dish.
Heat the 2 tablespoons of butter in a saucepan over moderate heat. Add the flour and mustard and cook, stirring until smooth, about 1 minute. Add the milk and cook over moderately low heat, stirring occasionally, until thickened, about 10 minutes. Add the cheese and stir until melted and completely smooth. Season the sauce with salt and pepper, and keep it warm over very low heat.
Meanwhile, in a medium pot of boiling well-salted water, cook the pasta until al dente; drain well. Add the pasta to the cheese sauce and stir to coat completely. Pour into the buttered gratin dish and top with the crumbled potato chips. Bake until bubbling and golden, about 30 minutes.