01 - Preheat oven to 375°F and line a baking sheet with parchment paper
02 - Slice each croissant horizontally, leaving the back edge intact to create a pocket
03 - Whisk together Dijon mustard, mayonnaise, and chives in a small bowl, season with black pepper
04 - Spread 1 teaspoon mustard mixture inside each croissant, layer with ham and Swiss cheese slice
05 - Gently press croissants closed and arrange on prepared baking sheet seam-side down
06 - Brush tops with beaten egg and sprinkle with grated Parmesan
07 - Bake for 15-18 minutes until golden brown, cheese melted and filling is heated through
08 - Let cool for 2-3 minutes before serving warm