Blanch the spinach. In the meantime, braise the onion in olive oil, along with the cut into slices steak and mushrooms. Dissolve the vegetable stock in a little water and add it to the onions, mushrooms and steak. Once the water evaporates, add the spinach as well, stir.
Add the grated cheese. Sprinkle with black pepper. Divide up the puff pastry in two, take a rectangular tray, pour cold water in it and place half of the puff pastry. Pour the filling on top, place the other half of the puff pastry.
Smear egg white along the sides and egg yolk on top.
Bake in a preheated oven.