Mix the pork with the garlic, lemon zest, coriander, thyme, verjuice and prunes in a non-metallic dish. Season well. Cover and marinate in the fridge for up to 12 hours.
Preheat the oven to 180C/Gas 4/fan oven 160C. Remove the pork and prunes from the marinade and set
aside separately. Pat the pork dry, then toss in the flour. Heat 3 tablespoons of the oil in a frying pan and brown the pork on all sides (you may have to do this in batches). Transfer the pork to a large, wide casserole dish.
Wipe out the frying pan, add the remaining oil and brown the onions or shallots on all sides. Remove and set aside. Add the marinade to the pan and bring to the boil. Cook for 2-3 minutes, then stir in the stock and sugar and bring back to the boil. Pour over the pork, cover and cook in the oven for 11⁄2 hours. (You can prepare up to this stage a day ahead. Cover and refrigerate overnight. Next day, skim off fat if necessary; reheat until bubbling before carrying on with the recipe. You may need to add a little more stock or water.)
About 20 minutes before the pork is ready, peel the pears and cut them into wedges, then remove the cores. Remove the pork from the oven and adjust the seasoning, adding more sugar if you like. Stir in the onions or shallots, pears and potatoes, cover the casserole and return to the oven for another 40 minutes or until the potatoes are just tender.
Uncover the dish and stir in the prunes, then stir in a little more stock if the casserole seems dry. Return to the oven for another 15-20 minutes, until the potatoes are fully cooked. Check the seasoning, then scatter the parsley and lemon zest over the top.