01 - Preheat oven to 350°F. Line a baking sheet with parchment paper.
02 - Slice each croissant horizontally without cutting all the way through, creating a hinged opening.
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 and brush tops with melted butter. Arrange on prepared baking sheet.
06 - Bake for 12 to 15 minutes until cheese is melted and croissants are golden and crisp.
07 - Serve warm while cheese is still melted.