Preheat the slow cooker if necessary; see the manufacturer's instructions. Heat the oil in a large frying pan, add the pieces of pork and fry over a high heat until browned on both sides. Lift the pork out of the pan with a slotted spoon and transfer to a plate.
Add the onion to the pan and fry, stirring, for 5 minutes or until lightly browned. Stir in the flour, then mix in the stock, orange rind and juice, plum sauce, soy sauce, star anise and chilli (if used). Season with salt and pepper and bring to the boil, stirring.
Transfer the pork to the slow cooker pot and pour the sauce over it. Cover with the lid and cook on low for 8–10 hours. Sprinkle with grated orange rind and serve with mashed potatoes mixed with steamed green vegetables.