Cut down the centre of the pork fillet to make a deep pocket, being careful not to cut all the way through. Push the shallots into the pocket all the way along and then rub the outside of the pork with olive oil and season well. Tie the pork along its length with butchers string to keep it together, tucking the whole sage leaves under the ties. Heat a little oil in a frying pan and brown the pork all over. Transfer to a roasting tin and cook for 25 minutes, or until a skewer poked into the centre comes out feeling hot. Leave to rest for 10 minutes.