Preheat your oven to 180°C (160°C fan/Gas Mark 4). Bring a large pot of salted water to the boil and cook the pasta for about 2 minutes less than the packet instructions state. It should be very al dente. Drain and set aside.

While the pasta is cooking, heat the olive oil in a large saucepan or casserole dish over a medium heat. Add the chopped onion and cook gently for 8-10 minutes until soft and translucent, but not browned. Add the minced garlic and cook for another minute until fragrant.

To make the sauce, melt the butter in the same pan with the onions and garlic. Stir in the plain flour to form a thick paste (this is called a roux). Cook for 1-2 minutes, stirring constantly. You can find excellent visual guides on how to make a perfect roux sauce if you're new to the technique.

Remove the pan from the heat and begin adding the milk a little at a time, whisking vigorously after each addition to ensure a lump-free sauce. Once all the milk is incorporated, return the pan to a medium heat and bring it gently to a simmer, stirring until the sauce has thickened. This should take about 5-7 minutes.

Once thickened, take the sauce off the heat. Stir in the mustard powder and the 250g of grated mature cheddar. I find that stirring the cheese in off the heat helps it melt smoothly without splitting. Season generously with salt and pepper to taste.

Add the drained, cooked pasta, the drained tuna (breaking it up into large flakes), and the sweetcorn to the cheese sauce. Stir gently until everything is evenly coated.

Pour the tuna pasta mixture into a large ovenproof dish (approximately 20cm x 30cm).

In a small bowl, mix the 50g of breadcrumbs with the extra 30g of grated cheddar. Sprinkle this mixture evenly over the top of the pasta.

Bake for 20-25 minutes, or until the top is golden brown and the sauce is bubbling up around the edges. Let it stand for 5 minutes before serving.