Preheat the oven to 375°F (190°C).
Spread Dijon mustard inside each croissant, if using.
Layer two slices of ham and two slices of cheese inside each croissant.
Brush the tops of the croissants with melted butter.
Place the croissants on a baking sheet lined with parchment paper.
Bake for 10–12 minutes or until the cheese is melted and the croissants are golden brown.
Serve warm and enjoy!