Bring a large pan of salted water to the boil. Add the pasta, boil for 5 minutes and drain.
Meanwhile, heat the milk in a small pan. In a separate, large saucepan, melt the butter and add the flour, beating quickly until it forms a smooth roux (base for the white sauce). Whisk in the hot milk a little at a time until it is all incorporated and bring to the boil. Turn down to a simmer, stir in the mustard and season with salt and pepper. Cook for 5 minutes, stirring often. Add the par-cooked pasta to the pan and cook, stirring, for a further 3-4 minutes. The starches in the pasta will thicken the sauce further as it cooks.
Preheat the grill to medium. Remove the sauce from the hob and stir in two thirds of the grated Cheddar cheese and a third of the Parmesan.
Transfer to an ovenproof dish, sprinkle with the remaining cheeses and grill for 5-7 minutes, until cheese is browned and the sauce is bubbling.
Serve with a crisp green salad or steamed seasonal vegetables.
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