Spread butter on one side of the bread slices.
In a medium pan, on medium heat, place bread slices butter side down.
Place cheese slices on both of the bread slices.
Place tomatoes on one slice of bread over the cheese.
Scoop a generous portion of the tuna filling and spread over other bread slice.
Flip the bread slice with tomato over the tuna filling to make a sandwich. Keep toasting in the pan until crust is crunchy. Serve right away.