Line a chopping board with cling film and place the pork on the board and score the meat lengthways with a sharp knife (not all the way through). Top with another sheet of cling film and flatten out the meat to a half inch thickness.
Line the inside of the pork with the guanciale.
Mix the stuffing and place on top of the guanciale.
Roll up the pork steak with the stuffing inside like a swiss roll, using the cling film to keep it firmly together.
Lay the slicced prosciutto on the board. Place the pork roll on top and roll this up, making sure the pork steak is completely encased in prosciutto.
Seal the stuffed pork on a hot pan on all sides.
Wrap the seared stuffed pork fillet in 2 sheets of tin foil. Roll very tightly but turn the ends of the tin foil upwards so the juices don’t leak out.
Place on a rack in a roasting tin and cook in the oven at 180°C for 1 hour.
Remove from tin foil and slice, pouring the juices over the meat.