Sear the beef:
Season the beef fillet generously with salt and pepper. Heat olive oil in a pan over high heat. Sear the beef on all sides for 2–3 minutes until browned. Remove and let cool slightly. Brush all over with mustard and set aside.

Prepare the mushroom duxelles:
In the same pan, melt butter over medium heat. Add onion, garlic, and mushrooms. Cook for 10–12 minutes until all moisture evaporates and the mixture is paste-like. Stir in thyme and season with salt and pepper. Let cool.

Wrap with prosciutto:
On a large piece of cling film, lay out overlapping slices of prosciutto. Spread the mushroom mixture evenly on top. Place the beef in the centre, then use the cling film to roll it tightly into a log. Chill for 15–20 minutes to firm up.

Wrap in pastry:
Roll out the puff pastry on a floured surface into a rectangle large enough to enclose the beef. Unwrap the beef from the cling film and place in the centre of the pastry. Fold over and seal the edges well. Trim any excess pastry, and brush all over with egg wash.

Chill and decorate:
Place the Wellington seam-side down on a lined baking tray. Chill for another 10–15 minutes. Score the top lightly with a knife for decoration, and brush again with egg wash.

Bake the Wellington:
Preheat the oven to 200°C (400°F). Bake for 40–45 minutes for medium-rare, or adjust time for preferred doneness. Rest for 10 minutes before slicing.

Slice and serve:
Cut into thick slices and serve hot with a red wine reduction, gravy, or creamy mashed potatoes.

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