Dice onion and capsicum and fry in the 2 tablespoons of butter until onion begins to go clear.
Turn the heat to medium and add the plain flour to the pan. Stir with a wooden spoon and as soon as it starts to thicken (20-30 seconds) start adding the milk. Continue stirring the mixture until there are no lumps of flour in the sauce (it will look very milky and thin).
Add the can of corn (including the juice) to the pan along with the tuna (strained). Add the grated cheese, salt and pepper to the pan and reduce the heat to low.
Start cooking the rice while the tuna mornay simmers. Keep stirring the sauce every 5 mins until you are ready to serve (the heat can be turned of 5 mins before serving).
You can add more cheese or pepper if you feel it needs a bigger flavour. I use a strong aged cheddar (12 months aged)
The taste of this dish is fantastic. Just a note: make sure you use the cheese that the recipe calls for, as i used low fat cheddar and it did not thicken so well and stuck to the tuna making it a bit chewy. (Not the recipes fault) Very yummy - 18 Feb 2012