Cut the squash in half lengthways and discard the seeds and fibres from the centre. Score the flesh with a sharp knife andplace in a shallow baking tray, flesh side up. Brush the flesh with olive oil and season with pepper. Roast for 40 minutes or until the flesh is tender.
Ten minutes before the end of cooking time, make the stuffing. Heat the butter in a pan and fry the mushrooms and leeks for 5 minutes more, stirring often. Remove the pan from the heat and stir in the ham, half the cheese and the rice vinegar.
Remove the squash from the oven and carefully spoon out the flesh, leaving a thin border to support thr skins. Mash the flesh and add to the stuffing along with the parsley. Return the skins to the baking tray and divide the stuffing between each one, pressing down lightly and piling up as necessary. Sprinkle with the remaining cheese, return to the oven and bake until the cheese melts and the filling is piping hot. Scatter with chives and serve immediately.