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, and chives if using. Season with black pepper to taste.
04 - Spread approximately 1 teaspoon of mustard mixture inside each croissant pocket. Layer one slice of ham and one slice of Swiss cheese inside.
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 15-18 minutes until croissants are golden brown, cheese is melted and bubbling, and filling is thoroughly heated.
08 - Allow croissants to cool for 2-3 minutes before serving to prevent burning from hot filling.