Preheat the oven to 400°F or 200°C
Cut each sheet of puff pastry in quarters and lay four on a baking sheet lined with parchment paper - if you end up with squares, I like to stretch it a bit to form a rectangle
Spread ½ tsp of dijon mustard on each of the puff pastry
Sprinkle a layer of shredded cheese on each piece, then ham, then cheese again
Sprinkle a pinch of herbs de provence over each of the pastry
Cover each pastry with the remaining pastry rectangle and seal by pinching the edges together
Beat the egg yolk and water and then glaze the top of each with a pastry brush
Using a knife, cut 2-3 diagonals across the top of the pastry to let steam escape during the baking process
Bake the feuillete on the middle rack in the oven until nice and golden brown, 20-25 minutes
Take it out of the oven and let it cool down completely before wrapping it up in parchment paper or a napkin in place in a ziploc bag