Although I do like grilled fresh tuna, don’t give me hot canned tuna as in casserole yech!![xx(] However, in a tuna melt the tuna usually doesn’t really get hot. The bread is usually toasted before the tuna is put on. Then the slice of cheese on top, it is just run under a broiler long enough to melt the cheese, if the tuna is real cold it won’t get hot at all, generally just to room temp.
Unfortunately finding a good tuna melt is becoming harder to do[:(]