Grate the cheese and add to a bowl (preferably glass - so it won't get scratched)

drain the fluid from the tuna can and add tuna to bowl

turn your toastie maker on to allow it time to warm-up

add the mayonnaise to the bowl and mix thoroughly with a fork

Butter the bread. Sandwich some of the mixture between two slices of bread and cook in toastie maker (orient butter-side to whatever works with your machine)

Note: You can keep-back some of the mixture in a refrigerator if required, or simply alter the amount of ingredients to suit.