Start by boiling the rigatoni in a pot of salted water until it’s al dente. Check the package for timing so it does not get mushy. Drain when ready.
Melt the butter in a pan and add the diced red onion. Sauté until soft and aromatic, about a few minutes, until the onion turns translucent.
Stir in the flour to the buttery onion mixture until it is well combined and forms a paste-like consistency. Slowly pour in the milk while whisking continuously until the mixture is smooth to avoid lumps.
Mix in the salt, pepper, and Dijon mustard. Fold in the cheddar until it is melted and creamy. If the sauce appears clumpy, whisk it more for a smooth texture.
Gently fold in the flaked tuna and sweetcorn into your creamy sauce, ensuring everything is well-coated without over-stirring.
In a large bowl, combine the pasta and sauce, making sure each piece of rigatoni is well-coated. If it seems too dry, add a splash of milk to loosen it up.
Pour the mixture into a greased baking dish, spreading it evenly.
Bake until golden. Let cool slightly before garnishing with fresh parsley.