01 - Preheat oven to 375°F and line a baking sheet with parchment paper.
02 - Slice each croissant horizontally without cutting completely through, creating a pocket for filling.
03 - Whisk together Dijon mustard, mayonnaise, and chives. Season with black pepper.
04 - Spread 1 teaspoon of mustard mixture inside each croissant, then layer one slice of ham and one slice of Swiss cheese.
05 - Press croissants closed, place on baking sheet, brush tops with beaten egg, and sprinkle with grated Parmesan.
06 - Bake for 15 to 18 minutes until golden brown, cheese is melted, and filling is hot throughout.
07 - Let cool for 2 to 3 minutes before serving warm.