All I had to do the next morning - brush the egg wash and bake in the oven for 20 minutes at moderate heat (140-160C).
Egg yolk, beaten with a pinch of salt
- Cut the pastry sheet into 4 squares.
- Place the grated cheese and ham in the centre.
- Fold the four corners into the centre.
- Place the pastries on a baking tray lined with baking paper. Put the tray into the fridge overnight or for 10 minutes (whilst oven is warming up).
- Brush with egg wash and place the tray into the hot oven. Bake for 20 minutes at 140-160C.