1. Cut the beef log into 2 large pieces approximately 400-450g each. Trim the logs and reserve the trimmings for the sauce.
2. Wrap each piece of beef tightly in a triple layer of cling film to set its shape, then chill overnight.
3. Remove the cling film, then quickly sear the beef fillets in a hot pan with a little olive oil for 30-60 seconds until browned all over and rare in the middle. Remove from the pan and leave to cool.
4. Finely chop the mushrooms and fry them in a hot pan with a little olive oil, thyme leaves and some seasoning. When the mushrooms begin to release their juices, continue to cook over high heat for about 10 minutes until all the excess moisture has evaporated and you are left with a mushroom paste.
5. Remove the mushroom paste from the pan and leave it to cool.
6. Cut the pastry in half, place it on a lightly floured surface and roll each piece into a rectangle large enough to envelop one of the beef fillets. Chill in the refrigerator.
7. Lay a large sheet of cling film on a work surface and place 4 slices of prosciutto in the middle, overlapping them slightly, to create a square. Spread half the mushroom paste evenly over the prosciutto.
8. Season the beef fillets, then place them on top of the mushroom-covered prosciutto.
9. Using the cling film, roll the prosciutto over the beef, then roll and tie the cling film to get a nice, evenly thick log. Repeat this step with the other beef fillet, then chill for at least 30 minutes.
10. Brush the pastry with the egg wash. Remove the cling film from the beef, then wrap the pastry around each ham-wrapped fillet. Trim the pastry and brush all over with the egg wash. Cover with cling film and chill for at least 30 minutes.
11. Meanwhile, make the red wine sauce. Heat the oil in a large pan, then fry the beef trimmings for a few minutes until browned on all sides. Stir in the shallots with the peppercorns, bay and thyme and continue to cook for about 5 minutes, stirring frequently, until the shallots turn golden brown.
12. Pour in the vinegar and let it bubble for a few minutes until almost dry. Now add the wine and boil until almost completely reduced. Add the stock and bring it to a boil again. Lower the heat and simmer gently for 1 hour, removing any scum from the surface of the sauce, until you have the desired consistency. Strain the liquid through a fine sieve lined with muslin. Check for seasoning and set aside.
13. When you are ready to cook the beef wellingtons, score the pastry lightly and brush with the egg wash again, then bake at 200°C for 15-20 minutes until the pastry is golden brown and cooked. Rest for 10 minutes before carving.
14. Meanwhile, reheat the sauce. Serve the beef wellingtons sliced, with the sauce and mashed potatoes.
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