01 - Preheat the oven to 350°F. Line a baking sheet with parchment paper.
02 - Using a sharp knife, slice each croissant horizontally, leaving a small hinge so they open like a book.
03 - In a bowl, mix together the ham, Gruyère, Dijon mustard, mayonnaise, chives, and black pepper until well combined.
04 - Gently open each croissant and evenly distribute the filling mixture inside.
05 - Place the stuffed croissants on the prepared baking sheet. Brush the tops with melted butter and sprinkle with Parmesan.
06 - Bake for 12-15 minutes, or until the croissants are golden and the cheese is melted.
07 - Serve warm.