Preheat oven to 220C (425F, gas mark 7).
Place prunes, walnuts (and brandin a bowl and mix, checking there are no pits.
Using a sharp knife make a long slit pocket running inside the middle of the meat. Stuff the prune mixture in, ensuring it is well packed in. Some of this mixture will flow out during cooking and this will enrich the onion gravy.
Mix 2 tbsps oil, marjoram, salt and pepper in a bowl. Rub mixture all over the meat. Heat a little oil in a frypan and brown the meat high heat on all sides to seal – however, if you’re using a skin on cut, do not brown the skin!
Place onion and any remaining oil in a small roasting tin. Pop the meat on top and roast on the top rack at 220C for 30 minutes to get the crackling going, then move to middle shelf and roast for a further 75 minutes at 180C (for loin only, baste occasionally with pan juices). Turn oven off and allow the meat to rest for another 15 minutes.
Serve warm with rice or bread and lashings of gravy. Make sure to use all the crackling during the first serving as that’s when it is at its best. For salads, try the red cabbage and mint slaw.