This storecupboard recipe is a student classic and is tasty enough for a midweek family meal too.
Preheat the oven to 180C/350F/Gas 4.
Cook the pasta in a saucepan of plenty of boiling water according to packet instructions. Drain.
Meanwhile, for the sauce, melt the butter in a saucepan over a low heat and stir in the flour with a wooden spoon. Cook for one minute, or until the mixture darkens slightly.
Slowly stir in the milk, stirring until smooth after each addition of milk. Stir in the mustard powder and continue to cook until the mixture thickens enough to coat the back of the spoon.
Remove the pan from the heat and stir in most of the cheese, and all the sweetcorn, peas and spring onion. Season, to taste, with salt and freshly ground black pepper. Stir the cooked pasta into the sauce.
Spoon the tuna into the bottom of an ovenproof dish. Pour over the pasta and sauce. Tap the dish gently so the sauce pours into every nook and cranny. Sprinkle over the remaining grated cheese.
Bake for 15-20 minutes, or until the sauce is bubbling and the top is a golden-brown.