Put the potatoes on to cook in a saucepan of lightly salted boiling water.
Slice the pork fillet into 8 equal pieces. Lay them flat and place a sage leaf on top of each one. Wrap slices of Parma ham around the pork pieces to enclose the sage leaves.
Heat the olive oil in a large frying pan and add the Parma-wrapped pork. Fry over a medium-high heat for 1-2 minutes, then turn over to seal and brown the other side for 1-2 more minutes.
Add the Marsala or wine, let it bubble up for a moment, then add the stock and mushrooms. Reduce the heat and simmer for 8-10 minutes, turning the pork once and stirring the mushrooms.
Meanwhile, cook the green beans for 4-5 minutes, until tender. Serve with the pork and new potatoes.