Season the fillets generously with the cracked pepper and salt. Heat the oil in a large frying pan or flameproof roasting tray until starting to smoke. Then, one at a time, sear the fillets until brown on all sides – about 6 mins per fillet. Leave the beef to cool slightly. Meanwhile, lay the prosciutto so that it’s overlapping on two large sheets of greaseproof paper.
Heat oven to 200C/fan 180C/gas 6. Brush the beef all over with mustard and place on the prosciutto. Wrap the prosciutto around the beef and secure with string.
Sit the fillets next to each other on a roasting tray and roast for 40 mins for medium or 50 mins for well done. Leave the beef to cool in the roasting tray. Dab the fillets with kitchen paper to remove any excess juices. Then either carve into thin slices and lay over two platters or place the fillets on the platters and serve with a carving knife and fork, so that guests can help themselves.