Chill can of tuna overnight or longer. Drain water.

Put tuna in a medium mixing bowl and shred it with your fingers so it isn't in large chunks.

Add celery, cilantro (if using it) and mayonnaise and mix thoroughly.

Heat a heavy skillet over medium high heat until it is very hot. It will take a few minutes. 

Spread the butter evenly on one side of the bread slices and turn them over. I put them on a cutting board so they don't mess up the countertop which does not go into the sink!

Place one slice of cheese, 1/3 of the tuna salad, a few slices of tomato, another slice of cheese and the other slice of bread, butter side on the outside on the cutting board. 

Put the sandwiches in the pan and let them saute for several minutes until they are toasty brown on the outside. Turn over and brown the second side. When the second side is brown, turn the heat down to low and let the sandwiches cook for a few minutes on each side until the cheese melts.

Remove the sandwiches to plates and let them rest for a minute or two to allow the cheese to solidify a bit before cutting the sandwiches. On the diagonal. Always on the diagonal!