Preheat your oven to 180ºC, then boil the pasta in salted water for 2 minutes less than the packet states. Chop the spring onion and set aside
Dice the onion and fry whilst the pasta is cooking, then add the butter. Once melted, add flour, stir in and cook for a minute. Add the milk gradually, stirring as you do
Stir in 200g cheese until it melts, then add mustard
Drain the pasta and add to a bowl, then stir the sauce through. Drain the sweetcorn and add along with spring onion, peas and tuna. Mix well, then tip into an oven proof dish, top with 150g cheese and bake for 22-25 minutes, then serve and enjoy!