Preheat your oven to 375°F (190°C).
Slice each croissant in half lengthwise.
Spread a thin layer of Dijon mustard on the inside, if using.
Place a slice of ham on the bottom half of each croissant.
Add a slice of cheese over the ham.
Place the top half of the croissant back on.
Arrange the filled croissants on a baking sheet.
Bake for about 15 minutes, or until the cheese is melted and the croissants are golden brown.
Remove from the oven and let cool slightly.
Garnish with fresh herbs before serving.