01 - Preheat oven to 375°F. Line a baking sheet with parchment paper.
02 - Slice each croissant horizontally, leaving the back edge intact to create a pocket for filling.
03 - Whisk together Dijon mustard, mayonnaise, chives, and black pepper in a small bowl until smooth.
04 - Spread approximately 1 teaspoon of mustard mixture inside each croissant pocket.
05 - Place one slice of ham and one slice of Swiss cheese inside each prepared croissant.
06 - Gently press croissants closed and arrange on the prepared baking sheet, seam-side down.
07 - Brush tops of croissants with beaten egg using a pastry brush. Sprinkle with grated Parmesan if desired.
08 - Bake for 15-18 minutes until croissants are golden brown, cheese is melted, and filling is hot throughout.
09 - Allow croissants to cool for 2-3 minutes before serving warm.