Spread Dijon mustard on one side of each bread slice.
Layer ham, Swiss cheese, arugula, and red onion (if using) on one slice of bread.
Top with the other bread slice, mustard side down.
If desired, spread butter on the outside of the sandwich and grill in a skillet over medium heat until golden brown and cheese melts, about 3 minutes per side.
Slice in half and serve.
Tips
For a warm sandwich, grill until cheese melts.
Pair with a pickle spear and potato chips.
Substitute honey mustard or whole-grain mustard for variety.