This is such a simple dish that the title pretty much explains all that you need to know. Nonetheless, I will help you out and give you some instructions on how to make the perfect one.
Take two slices of bread.
Toast under a hot grill on one side.
Take out from under the grill. Now turn the grill to a low/medium heat.
Spread half a tin of tuna onto each slice, on the un-toasted side.
Sprinkle with finely chopped spring onions.
Place slices of cheese on the top of the tuna and onion mixture.
Place under the grill, which should now be on a low-ish heat.
Watch it carefully until done to your liking, The cheese should be melted, bubbling and just starting to brown a tiny bit.
This post has been bought to you via my mobile phone from a packed commuter train, so that is why you only get one picture today!