01 - Set oven to 350°F and line a baking sheet with parchment paper.
02 - Slice each croissant horizontally without cutting completely through, creating a hinge for filling.
03 - Spread a thin layer of Dijon mustard inside each croissant if desired.
04 - Place one slice of ham and one slice of Swiss cheese inside each croissant.
05 - Close croissants gently, brush tops with melted butter, and arrange on the prepared baking sheet.
06 - Bake for 12-15 minutes until cheese melts and croissants are golden and crisp.
07 - Remove from oven and serve immediately while warm.