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Bring a pan of water to the boil. Add the pasta and cook, following pack instructions, until tender. Add the peas for the final 3 mins cooking time.
Meanwhile, melt the butter in a pan over a medium heat. Stir in the flour and cook for 2 mins. Add the milk, whisking constantly, then slowly bring to the boil, stirring often, until sauce thickens. Remove from the heat, add the mustard and season well.
Heat the grill to medium. Drain the pasta and peas, then return to the pan and stir in the tuna, spring onions, sweetcorn and sauce. Tip into a shallow baking dish, top with the cheddar and cook under the grill for 5 mins or until golden and bubbling.