Preheat the oven to 180℃
Boil the pasta in salted boiling water for two minutes less than the packet states.
Meanwhile, fry the diced onion over a medium to high heat for a few minutes until it starts to soften.
Turn the heat down to a medium, then add the butter, once it's melted add the flour and cook it for one minute.
Gradually pour in the milk, stirring often until you have used it all up. Cook until it starts to thicken.
Turn off the heat then stir through the grated cheese and the dijon mustard. Stir until the cheese has melted. Taste for seasoning.
Drain the pasta in a colander, then transfer it to a large mixing bowl.
Pour over the cheese sauce, add the sweetcorn, spring onions, peas and tuna. Mix it well then tip it into an oven proof dish.
Top it with more grated cheese and bake it for 20-25 minutes, until it is bubbly and golden.
Dish it up and enjoy.