Pre heat the oven to 180 degree C / 356 F, I use an electric fan oven, please adjust according to your own oven.
Cook the pasta according to the packet instructions minus 2 or 3 minutes. You want the pasta to slightly under done.
While the pasta is cooking make the sauce. Melt the butter on a medium saucepan. Once melted add the flour, stir for about 1 minute. Then pour in the milk, I add 100ml at a time. I use a whisk to stir in the milk. This has always worked to achieve a really smooth sauce.
As soon as all the milk has been added, stir in the mustard and a small pinch of the black pepper. When the sauce has thickened and starts to bubble turn off the heat then add 100g of the cheese and stir until the cheese has melted.
Drain the pasta, then tip it into a large, oven proof, serving dish. Add the tuna, sweetcorn, spring onions and most of the parsley to the pasta. Pour the sauce over the pasta then mix well.
Top the pasta with the bread crumbs, the remaining cheddar, salt and pepper and the parmesan. Bake in the oven for about 20 minutes until the top is golden brown and the cheese is bubbling.
Remove the pasta bake from the oven and let it sit for a minute or two, it will be piping hot. Sprinkle the remaining parsley over the pasta bake for serving. Serve with a green salad.