In a small bowl, mix together the mayonnaise and Dijon mustard. Set aside.
Heat a skillet over medium heat and melt 1 tablespoon of butter.
Spread the mayonnaise mixture evenly on one side of each slice of bread.
Place two slices of bread, mayonnaise side down, in the skillet. Layer each with sliced turkey and a slice of Swiss cheese.
Top with the remaining slices of bread, mayonnaise side up.
Cook for about 3-4 minutes, or until the bottom is golden brown. Carefully flip the sandwiches and cook for another 3-4 minutes until the other side is golden and the cheese is melted.
Remove from the skillet and let cool for a minute before slicing in half. Add lettuce and tomato if desired.