Slice each pretzel bun in half horizontally.
Spread 1 teaspoon Dijon mustard on the cut side of each bun half.
Layer 2 slices of deli ham and 1 slice of Swiss cheese on the bottom half of each bun.
Close the sandwich with the top bun half.
Heat a skillet over medium heat and melt butter.
Place sandwiches in the skillet and toast for about 2-3 minutes per side, pressing lightly, until the cheese is melted and the pretzel buns are golden and glossy.
Remove sandwiches from the skillet and let rest for 1 minute before slicing.
Serve two halves stacked slightly offset on a wooden board with a small ramekin of Dijon mustard on the side.