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 mixing bowl, combine the chopped ham, shredded Gruyère cheese, Dijon mustard, mayonnaise, fresh chives, and black pepper. Mix until evenly distributed.
04 - Gently open each croissant and evenly distribute the filling mixture inside, pressing lightly to adhere.
05 - Place the stuffed croissants on the prepared baking sheet. Brush the tops with melted butter and sprinkle with grated Parmesan cheese.
06 - Bake for 12-15 minutes, or until the croissants are golden brown and the cheese is melted and bubbly.
07 - Remove from oven and serve warm while the cheese is still melted.