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.