Heat the oil in a large, flameproof casserole dish. Season the pork, and brown it in 2 or 3 batches until golden, about 5 mins per batch.
Set aside in a bowl. Stir the coriander stalks, chilies and anise to the dish, sizzle for 1 min, stirring often, then mix in the sugar and tomato purée.
When these have melted and it looks a bit like barbecue sauce, return the pork and its resting juices to the dish, along with the fish sauce and stock. Tuck in the pineapple chunks here and there.
Once simmering, cover the hotpot, leaving a small gap for steam to escape, and transfer to the oven for 2 hrs. With 30 mins to go, skim some of the fat from the top (this tends to seal the liquid in, making it more difficult for the sauce to reduce), then return to the oven.
The pork will be meltingly soft when ready. If the sauce is thin, lift the pork into a warmed serving dish, put the casserole dish on the hob and simmer until the sauce has slightly thickened. Leave to cool for 5 mins, taste for seasoning, pour over the pork and sprinkle over the coriander leaves.