Preheat the slow cooker if necessary; see the manufacturer's instructions. Put the duck and belly pork into the base of the slow cooker pot. Tuck the onion and apple between the pieces of meat and add the thyme.
Pour the stock and cider into a saucepan and add plenty of salt and pepper. Bring to the boil, then pour into the slow cooker pot. Cover with the lid and cook on high for 5–6 hours or until the duck and pork are cooked through and tender.
Lift the meat out of the slow cooker pot with a slotted spoon and transfer to a large plate, then leave to cool for 30 minutes. Peel away the duck skin and remove the bones. Shred the meat into small pieces and discard the thyme sprigs. Scoop out the apple and onion with a slotted spoon, finely chop and mix with the meat, then taste and adjust the seasoning, if needed.
Pack the chopped meat mix into 4 individual dishes or small 'le parfait' jars and press down firmly. Spoon over the juices from the slow cooker pot to cover and seal the meat. Leave to cool, then transfer to the refrigerator and chill well.
When the fat has solidified on the top, cover each dish with a lid or clingfilm and store in the refrigerator for up to 1 week. Serve the rillettes with warm crusty bread, a few radishes and pickled shallots, if liked.