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 - In a small bowl, whisk together Dijon mustard, mayonnaise, and chives. Season with black pepper.
04 - Spread approximately 1 teaspoon of mustard mixture inside each croissant. Layer one slice of ham and one slice of Swiss cheese inside each pocket.
05 - Gently press croissants closed and arrange on prepared baking sheet, seam-side down.
06 - Brush tops of croissants with beaten egg using a pastry brush. Sprinkle with grated Parmesan if desired.
07 - Bake for 15 to 18 minutes until croissants are golden brown, cheese is melted, and filling is heated through.
08 - Allow croissants to cool for 2 to 3 minutes before serving.