01 - Preheat your oven to 375°F (190°C). Line a baking sheet with parchment paper.
02 - Slice each croissant horizontally, but not all the way through, creating a pocket for the filling.
03 - In a small bowl, mix together the Dijon mustard, mayonnaise, and chives (if using). Season with black pepper to taste.
04 - Spread about 1 teaspoon of the mustard mixture inside each croissant. Layer one slice of ham and one slice of cheese inside each croissant.
05 - Gently press the croissants closed and place them on the prepared baking sheet. Brush the tops with the beaten egg. Sprinkle with grated Parmesan, if desired.
06 - Bake for 15–18 minutes, until the croissants are golden brown, the cheese is melted, and the filling is hot.
07 - Let cool for 2–3 minutes before serving.