-
Preheat the oven to 400°F
-
Use a large saucepan to cook the macaroni in lightly salted boiling water according to pack instructions. Drain carefully in a colander.
-
Melt the butter in another saucepan and stir in the flour and mustard to make a paste. Cook over a low heat for 1 minute, stirring all the time.
-
Slowly whisk in the milk a little at a time until smooth, then simmer the sauce until thickened.
-
Stir two-thirds of the cheese into the sauce, then mix in the macaroni and season to taste.
-
Transfer the macaroni cheese to a baking dish, scraping out all the sauce with a wooden spoon, then level the top flat.
-
Sprinkle over the crushed chips. Cover with the remaining cheese and cook in the oven for 20 minutes until golden and bubbling.