Chop the onion and sauté briefly in oil. Add the spinach leaves and cook over a medium heat until it collapses slightly. Take off the stove. Finely grate the hard cheese and stir into the spinach mixture. Season the mixture with salt, pepper and nutmeg.
Roll out the puff pastry lightly and divide into 6 rectangles. Divide the spinach mixture into the 6 corners and wrap or roll up the parcels (ideally in triangles). Mix the egg and brush the puff pastry with it. Bake in the oven at 200 ° C for 20 minutes.