Preheat the oven to 180 degrees C. Heat the oil in a large flameproof casserole dish, then briefly fry the pork steaks on both sides until lightly browned. Add the plums, spring onions, sweet potatoes and water chestnuts to the casserole dish, then stir everything together.
Put the ginger into a small bowl. Add the garlic, chilli if using, sugar, soy sauce, vinegar, wine or sherry and the five-spice powder. Mix well, then spoon the mixture over the ingredients in the casserole dish. Cover with the lid, then cook in the oven, stirring occasionally, for about 45 minutes until the meat is tender and the sauce has thickened.
Taste the sauce and add more sugar or vinegar if needed, depending on the flavour of the plums, to give a good balance of sweet and sour tastes. Scatter the chopped coriander over the top and serve.