For the marinade, combine the peppercorns, salt, herbs, garlic and sugar in a food processor, then gradually add the oil and vinegar.
Make six incisions in the piece of pork and carefully press half a prune and half an anchovy fillet into each slit. Rub the meat with the marinade and leave to marinate in a cool place for at least three hours or ideally overnight.
Preheat the oven to 150C/300F/Gas 2. Wipe off the marinade and brown the pork off in half the oil and half the butter, until brown on all sides.
Remove the pork and set aside. Add the remaining butter and oil to the pan and heat gently. Once foaming, add the onions and chopped sage.
Allow to cook over a gentle heat until the onion is soft and golden-brown. This will take about 20 minutes.
Add the flour and continue to cook for five minutes, stirring regularly.
Place the pork back on top of the onion slices in the casserole.
Add the marinade, stout and chicken stock to the pan, bring to the boil and cover the casserole.
Cook in the oven for about four and a half hours until the meat is very tender.
Slice the meat into large chunks. Keep warm.
Pour all the pan juices into a food processor and blend until smooth. If the liquid is too thin, boil vigorously to reduce.
Pour over the meat.
Serve with roast carrots, roast parsnips and mashed potatoes.