1. Cut the white mushrooms and the portobello mushrooms by hand and place them in the blender's bowl. Also add fresh thyme leaves, a clove of garlic finely chopped and the fresh onion. Mash well.
2. At the same time, heat a ply pan and add olive oil. We shield the fillet well from all sides. Once the fillet is shielded, set it aside to rest.
3. Finely chop the white onion and sauté in the same pan that shielded the fillet by adding little olive oil. Once the onion withers, add the mushroom mixture from the blender. Sauté well until all the liquids from the mushroom evaporate. At the end add chopped parsley and truffle oil to flavor the mixture.
4. Spread the mixture on absorbent paper to dry completely.
5. With a brush, brush the fillet with dyzon mustard on all sides and add plenty of freshly ground pepper.
6. Spread the film on your workbench in a size that can cover and wrap the fillet.
7. Place the 6 slices of prosciutto, one joined with the other at the top and then the remaining 6 at the bottom, creating a rectangular shape. 
8. Place the mushroom mixture on the prosciutto and spread with a ladle to go everywhere.
9. Then  take the fillet and place it on the edge of the rectangle that created with the prosciutto and the mushroom mixture.
10. With care take the edges of the film and start wrapping the fillet. Try to wrap as carefully and tightly so to have a uniform role in which if cut to distinguish all the textures of the recipe. 
11. Once we wrap it we pierce the edges with the nose of a knife to leave the air and place in the refrigerator for at least 30 minutes.
12. When half an hour has passed, open the puff pastry and spread it on the workbench. The puff pastry must be at cooler temperature and have a rectangular shape large enough to be able to wrap the fillet in. If it is too thick, with a rolling pin we open it about half a centimeter wide.
13. At the same time in a small bowl, join an egg and a yolk and mix well.
14. We take the roll of fillet from the refrigerator and remove the plastic wrap. Place it on the edge of the puff pastry as we did before in the stage where we wrapped the fillet in prosciutto.
15. With a brush, brush all the edges of the puff pastry with the egg. Start to wrap carefully and as soon as we cover the fillet with the puff pastry we press the edges on the sides and turn inwards to close.
16. Then spread again on our workbench a large piece of film and wrap tightly the roll that we created with the puff pastry giving it a nice cylindrical shape.
17. Place again in the refrigerator to tighten for about 1 hour.
18. Preheat the oven to 220 degrees C.
19. When the 1 hour has passed, remove the film from the roll and place in a baking pan.
20. Spread the roll very well with the egg on all sides and with a sharp knife create light diagonal inlets at the top of the roll.
21. Add salt on top of the puff pastry and leaves of fresh thyme.
22. Bake at 220 degrees for 20 minutes and then lower to 180 degrees for 10 minutes. In this way achieve a moderate baking. (if we prefer a more well-cooked result, at the stage where we shield our fillet we place for a while in the oven to bake more)
23. Let it rest for a while. With a serrated knife cut into wide pieces and serve.

A recipe by Stavri Georgiou