In a small bowl, mix together the mayonnaise and Dijon mustard. Spread this mixture evenly on one side of each slice of bread.
Layer the turkey breast evenly on two of the slices of bread, then top with a slice of Swiss cheese.
Place the remaining slices of bread on top, mayonnaise side down, to form two sandwiches.
Heat a large skillet over medium heat and melt the butter. Once melted, place the sandwiches in the skillet.
Cook for about 3-4 minutes on each side, or until the bread is golden brown and the cheese is melted. Press down gently with a spatula for even cooking.
Remove from the skillet and let cool for a minute before slicing in half. Add lettuce and tomato if desired.