1. Wash the mushrooms—make sure to wash them shortly before preparing the recipe. You can dry them using a salad spinner.
   
2. Cut off the ends of the stems and finely chop the mushrooms, including the stems; you can use a food processor for chopping.
   
3. In a skillet, melt the butter over medium heat and add the onion to soften; add the chopped mushrooms to the sautéed onion and cook over low heat until all the liquid evaporates.
   
4. When all the liquid from the mushrooms has evaporated, add parsley, salt, and pepper, and mix well. Turn off the heat once all the seasoning is incorporated.
   
5. Season the fillet on both sides with salt and pepper to taste. Heat olive oil in a very hot pan and sear the fillet on both sides, avoiding piercing it with a fork or knife.
   
6. Once done, wrap the fillet in a cloth and dry it well.
7. Keep the wrapped fillet in the refrigerator for about 15 minutes.

Assembly

1. On a flat surface, lay out a sheet of plastic wrap. Arrange the slices of ham side by side on the plastic wrap, ensuring there are no gaps between them. Spread the mushroom paste over the ham.
   
2. Place the fillet on the mushroom paste, and using the plastic wrap, roll the fillet tightly, ensuring the ends are secure. Keep in the refrigerator for approximately 30 minutes.
   
3. Spread flour on a flat surface and roll out the puff pastry with a rolling pin. Brush the pastry with egg yolk, remove the fillet from the refrigerator, unwrap the plastic wrap, and wrap the fillet with the puff pastry, sealing the sides.
   
4. If desired, decorate the top with strips of pastry. This makes slicing easier when it’s done. Place in the refrigerator for another 30 minutes.
   
5. Remove the beef from the refrigerator, brush the top with egg yolk, and bake in a preheated oven at 180ºC for about 40 minutes.

Sauce

1. In the same pan used for the fillet, add a drizzle of olive oil and sauté the onion and garlic. Add rosemary, thyme, and bay leaves to taste.
2. Crush the white peppercorns with a knife and add them to the sauté. Pour in the balsamic vinegar and port wine and let it reduce until almost dry.
3. Add the red wine and reduce until three-quarters of the total liquid remains. Blend the sauce in a blender with the cold butter and season with salt.

Final Touch

Reheat the sauce. Remove the Beef Wellington from the oven, slice it, and arrange it on plates. Pour the sauce over the top. If desired, garnish with fresh herbs and serve with gratin potatoes.