Sear the beef fillet:
Season the fillet with salt and pepper. Heat olive oil in a pan over high heat. Sear the beef on all sides until browned (about 2–3 minutes per side). Remove from heat, brush with mustard, and let it cool completely.

Prepare the mushroom duxelles:
In the same pan, add butter, garlic, onion, and chopped mushrooms. Cook over medium heat, stirring, until all moisture evaporates and the mixture is dry and paste-like. Season and cool.

Assemble the Wellington:
On a sheet of cling film, lay out prosciutto slices slightly overlapping. Spread the mushroom mixture over the prosciutto. Place the cooled beef on top, and using the cling film, roll tightly into a log. Chill for 15 minutes.

Wrap in pastry:
Roll out puff pastry on a floured surface. Remove the cling film and place the beef in the centre. Fold the pastry over and seal edges. Trim excess and pinch to close. Chill again for 10 minutes if needed.

Egg wash and decorate:
Place the wrapped Wellington seam-side down on a lined baking tray. Brush with beaten egg and decorate with pastry cutouts if desired. Brush again with egg wash.

Bake:
Preheat oven to 200°C (390°F). Bake for 40–45 minutes for medium-rare (internal temp ~50–52°C). Rest for 10–15 minutes before slicing.

Serve:
Slice into thick rounds and serve with roasted vegetables, gravy, or red wine jus.

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