Boil a large pan of water, add the pasta and cook for 10 minutes.
Whilst the pasta is cooking, melt the butter in a saucepan, add in the onion and cook on low for 6-8 minutes until the onions are very soft. Turn up the heat to medium and stir in the flour. Cook for 1-2 minutes. Slowly add the milk, using a whisk to stir (just stir, don’t whisk) it in. Once all the milk is incorporated, continue to heat whilst stirring until the sauce thickens. Then stir in 2/3 of the cheese until melted. Season with black pepper and a little salt if required (the cheese is quite salty though).
Drain the pasta and pour into a 20x30cm baking dish. Pour over the white sauce, then add the tuna, sweetcorn, petit pois and all but 1 tbsp of parsley. Mix together, then sprinkle on the remaining cheese.
Bake in the oven for 15-20 mins until the cheese is golden brown. Top with the reserved parsley before serving.