Put a small skillet (with lid) over medium heat.
Butter one side of each slice of bread.
Smear mustard on the unbuttered side of the bread slices.
Arrange cheese slices over one mustard-covered slice of bread.
Top cheese with tomato slices.
Cover with the other slice of bread.
Place the sandwich in the skillet – heat until bread is golden brown and the cheese is melty.
Slice diagonally and serve.