Preheat your oven to 375°F (190°C) and line a baking sheet with parchment paper.

In a small bowl, whisk together the egg, milk, Dijon mustard, salt, and pepper until well combined.

Slice each croissant in half horizontally, creating a top and bottom piece.

Spread a thin layer of the egg mixture on the inside of each croissant half.

Layer the sliced ham evenly on the bottom halves of the croissants, followed by a generous portion of grated cheese.

Replace the tops of the croissants and place them on the prepared baking sheet.

Brush the tops of the croissants with the remaining egg mixture to give them a beautiful golden color while baking.

Bake in the preheated oven for 15-20 minutes, or until the croissants are puffed and golden brown.

Remove from the oven and let cool slightly before serving. Garnish with fresh herbs if desired.