Cook the macaroni in boiling, salter water for about 8 minutes until slightly tender and then drain.
Whilst the macaroni is cooking add the milk and corn flour mixture into a medium sized saucepan. Bring the mixture to the boil, then turn it down to simmer whilst you stir it constantly with a wooden spoon to make a white sauce.
Once the sauce has thickened add the drained macaroni, tuna and peas and combine well together.
Pour the contents of the saucepan into an oven proof dish, sprinkle with the cheese and place onto the middle shelf of your oven.
Bake for approximately 15 – 20 minutes until the cheese has browned slightly and is bubbling.