Beat the egg and whisk with a dash of milk and a little salt.
Roll out the puff pastry and prick several times with a fork. Brush with the egg mixture and top with ham and cheese (you can also sprinkle some parmesan on top). Fold it up and press the edges firmly in place.
Brush the outside of both sides with the remaining egg mixture, prick the surface again. Bake in the preheated oven at 180 ° C hot air plus (strong bottom heat) on the lower rack for about 20 - 25 minutes.
After baking, let cool for a few minutes, cut into pieces and serve.