01 - Preheat oven to 375°F. Line a baking sheet with parchment paper.
02 - Slice each croissant horizontally without cutting completely through, creating a pocket for filling.
03 - In a small bowl, combine Dijon mustard, mayonnaise, and fresh chives. Season with freshly ground black pepper and mix well.
04 - Spread approximately 1 teaspoon of the mustard mixture inside each croissant pocket.
05 - Layer one slice of cooked ham and one slice of Swiss cheese inside each prepared croissant.
06 - Gently press croissants closed and place them seam-side down on the prepared baking sheet.
07 - Brush the tops of croissants with beaten egg using a pastry brush. Sprinkle grated Parmesan cheese evenly over the tops if desired.
08 - Bake for 15 to 18 minutes until croissants are golden brown, cheese is melted and bubbly, and filling is thoroughly heated.
09 - Let stuffed croissants cool for 2 to 3 minutes on the baking sheet before serving warm.