Generously season the beef with salt and pepper on all sides, pressing the seasoning in as you shape it into an even cylinder with your hands.

Heat a cast-iron skillet over medium-high heat, then add the oil.

Sear the beef fillet for 1 minute on each side until it’s browned evenly.

Place the beef on a sheet pan and pour any pan juices over it. While it's still hot, brush the fillet with 1 tablespoon of Dijon mustard to coat it, then let it cool.

Chop the mushrooms finely, or use a food processor if you prefer—just be careful not to overprocess them into a paste.

Warm 2 tablespoons of olive oil and a generous pat of butter in a large pan over medium heat, letting the aromas fill your kitchen as they melt together.

Add the chopped mushrooms, thyme, garlic, and parsley. Cook for 10 minutes, stirring often, until the mushrooms are soft, fragrant, and most of their moisture is gone.

Season the mushroom mixture with salt and pepper, then let it cool.

In a small bowl, mix the breadcrumbs, rolled oats, salt, black pepper, garlic powder, dried parsley, and Parmesan cheese (if using).

Stir until everything is well combined.

Lay two pieces of plastic wrap on a large cutting board, overlapping slightly to create a stable base.

Arrange the prosciutto slices on top, ensuring they overlap slightly to form a neat double row.

Evenly spread half of the cooled mushroom duxelles over the prosciutto, ensuring full coverage.

Sprinkle a thin, even layer of the breadcrumb mixture over the mushrooms to add texture.

Carefully place the beef fillet on top, then coat it with the remaining mushroom mixture and finish with a final layer of breadcrumbs. Proceed to wrap the layers tightly and neatly.

Use plastic wrap to tightly roll the prosciutto around the beef, mushrooms, and breadcrumbs, forming a compact, sausage-like shape.

Twist the ends of the plastic wrap to secure it, then refrigerate the wrapped beef to help it keep its shape while you prepare the pastry.

Lightly dust your work surface with flour to prepare for rolling.

Roll out one-third of the puff pastry into a strip approximately 7 x 12 inches and place it carefully on a parchment-lined baking sheet.

Next, take the remaining pastry and roll it into a larger rectangle, about 11 x 14 inches, ensuring it is evenly shaped and ready for use.

Remove the chilled beef fillet from its plastic wrap and place it in the center of the smaller puff pastry strip.

Brush the edges of the pastry and the beef fillet with egg wash (made by mixing 2 egg yolks with 1 teaspoon of water).

Lay the larger pastry sheet over the beef and press down around the edges to seal it tightly.

Trim the edges of the pastry, leaving a 1½-inch border. Use the back of a fork or the handle of a spoon to press and seal the edges firmly.

Evenly brush the entire surface of the pastry with a layer of egg wash.

Using the back of a knife, gently score diagonal lines across the top for decoration, ensuring not to cut through the pastry.

Chill the prepared Wellington for at least 30 minutes, or up to 24 hours.

Once ready to bake, preheat the oven to 400°F.

Apply a final layer of egg wash to the Wellington.

Bake for 20-25 minutes for medium-rare, or approximately 30 minutes for medium, until the pastry is golden brown and crisp.

Allow the Wellington to rest for 10 minutes before slicing and serving.