Preheat the oven to 230°C. Mix together the prunes, walnuts and brandy in a bowl.
Cut a deep, wide "pocket" through the middle of the joint - try not to go all the way through - then push the stuffing firmly into it.
Mix together the oil, herbs & seasoning then rub it all over the joint.
Spread the onion slices over a small roasting tin and dribble over the 1 Tbsp oil
Place the pork on top of the onion Put into the oven for 20 minutes then lower the temperature to 175°C and cook for about another 60 minutes, basting occasionally.
Allow the joint to rest, covered, on a carving plate while you use the juices in the pan to create a delicious gravy!