Season the beef generously with salt and pepper then heat 1 tbsp olive oil in a large frying pan over a high heat. Sear the beef on all sides until nicely browned (about 2-3 minutes on each side), don’t forget the ends too.
Remove the beef from the pan and pop on a plate to cool a little before brushing all over with the Dijon, then leaving to cool completely.
Pulse the mushrooms in a food processor with the shallot and thyme leaves until finely chopped.
Add the butter to the pan you browned the beef in and cook the mushroom mix over a medium high heat until all the moisture has evaporated and the mixture is dry (about 10-15 minutes). Season with salt and pepper, then set aside to cool.
Lay a large sheet of cling film on your work surface and arrange the prosciutto slices on the film, overlapping slightly, to form a rectangle large enough to wrap around the beef.
Spread the cooled mushroom mixture evenly over the prosciutto then place the beef in the middle and use the cling film to roll the prosciutto tightly around the beef. Twist the ends of the film to seal it.
Chill in the fridge for at least 30 minutes.
Roll out the puff pastry into a rectangle large enough to fully enclose the beef (about 30cmx40cm). Remove the beef from the cling film and place it in the centre of the pastry.
Brush the edges of the pastry with beaten egg, then fold over and seal tightly. Trim any excess pastry then brush the outside with more egg wash. Chill for 20 minutes.
Preheat the oven to 200℃ fan/220℃/Gas Mark 7/390F and place a baking tray in the oven to get hot.
Place the wellington on a large piece of parchment, brush once more with the beaten egg, then use a small sharp knife to lightly score a pattern onto the pastry, making sure not to cut all the way through, or decorate with pastry cut outs, sticking them on with egg wash.
Carefully place the wellington on the hot tray and bake for 35–40 minutes for medium-rare, then leave the wellington so rest for 10 minutes before slicing.
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