After playing around with this for a while I've discovered this is the best method.
This is incredibly tasty and almost anyone can eat it since EVERYTHING can be changed for something else!
I love adding tomatoes or sweetcorn to add a little bit extra.
Turn on your grill on a medium to high setting.
Toast 2 slices of bread (I use the toaster cos its quicker.
Layer your tuna on one of the slices of toast. The thicker the tuna, the longer it will take to warm through.
Put the tuna-toast under the grill. Leave the other toast slice where it is for now.
Grill until the tuna is warm through.
Use this time to slice your cheese and whatever else you're putting in. I'm just using Applewood Smoked cheese this time. 4 slices is plenty for my bread.
When the tuna is warm, take it out of the grill and lay the cheese on top.
Return to the grill until the cheese has started to melt.
Place the other slice of toast on top in the grill to warm it through.
Careful not to burn yourself!!
Take the sandwich out, cut and enjoy!