Slice pretzel buns in half horizontally.
Spread Dijon mustard evenly on the bottom halves of each bun.
Layer two slices of sliced deli ham on each bottom bun half.
Top ham with one Swiss cheese slice per sandwich.
Place the top bun halves over the cheese.
Heat a skillet over medium heat and melt butter.
Place sandwiches in the skillet and toast until buns are golden brown and cheese is melted, about 4-5 minutes per side.
Remove sandwiches from skillet and let rest for 1 minute.
Cut sandwiches in half and stack the halves slightly offset on a wooden board.
Serve with a small ramekin of Dijon mustard on the side.