Cook the Pasta: Preheat your oven to 200°C/180°C Fan/Gas Mark 6 (400°F). Bring a large pot of salted water to a boil and cook the pasta according to the package instructions until al dente. Drain and set aside.

Make the Cheese Sauce: In a medium saucepan, heat the butter over medium heat until melted. Add the flour, stirring constantly for 1-2 minutes to create a smooth roux. Slowly pour in the milk while whisking to prevent any lumps, and continue to cook until the sauce thickens and can coat the back of a spoon. Stir in the mustard powder and half of the shredded cheddar cheese, mixing until the cheese has fully melted and the sauce is velvety smooth.

Prepare the Filling: In a spacious bowl, mix together the cooked pasta, corn, peas, chopped spring onions, and drained tuna. Pour the creamy cheese sauce over the ingredients and carefully toss until everything is evenly coated.

Assemble and Bake : Spoon the mixture into a generously sized baking dish that’s safe for the oven. Evenly distribute the remaining shredded cheddar on top for a cheesy finish. Bake in the preheated oven for 20–25 minutes, or until the top is golden and bubbling.

Serve and Enjoy: Remove from the oven and let the bake rest for a few minutes before serving.