Crispy Topped Macaroni Cheese

Ingredients
500 g macaroni
40 g butter
40 g plain flour
1 tsp mustard powder
600 ml milk
175 g full flavour cheddar cheese, grated
25 g potato crisps, crushed
Salt and ground white pepper

Method
Preheat the oven to 200C, 400F, Gas 6. 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 crisps. Cover with the remaining cheese and cook in the oven for 20 minutes until golden and bubbling.