Preheat oven to 375°F (190°C) and line a baking sheet with parchment paper. Ensure croissant dough is at room temperature.
Separate each triangle of dough or cut sheets into triangles according to package instructions.
Place a slice of baked ham on each triangle of dough, folding slightly if necessary.
Add a slice of cheese on top of the ham for a gooey, savory filling.
Spread a thin line of Dijon mustard over the ham before adding cheese (optional).
Roll each triangle from the wide base to the tip to form a crescent shape, tucking ends underneath.
Beat the egg and brush over the tops of the croissants for a golden-brown finish.
Bake 15–20 minutes until croissants are puffed, golden, and cheese is melted.
Cool for 2–3 minutes, sprinkle with parsley or chives, and serve warm.