01 - Preheat oven to 375°F. Line a baking sheet with parchment paper.
02 - Slice each croissant horizontally, but not all the way through, creating a pocket for filling.
03 - In a small bowl, mix together Dijon mustard, mayonnaise, and chives if using. Season with black pepper to taste.
04 - Spread about 1 teaspoon of the mustard mixture inside each croissant pocket.
05 - Layer one slice of ham and one slice of Swiss cheese inside each croissant.
06 - Gently press the croissants closed and place them on the prepared baking sheet.
07 - Brush the tops of the croissants with the beaten egg. Sprinkle with grated Parmesan cheese if desired.
08 - Bake for 15-18 minutes until croissants are golden brown, cheese is melted, and filling is hot.
09 - Let cool for 2-3 minutes before serving to allow filling to set.