Cut the squash in half, spoon out the seeds and discard. Using a sharp knife, score the flesh of the squash in a criss-cross pattern, so that the heat can get to the interior. Place the squash halves, cut sides facing upwards, into an ovenproof dish, dot with the butter and season well with salt and freshly ground black pepper. Roast in the oven for one hour.
In the meantime, make the stuffing. Heat the butter in a large shallow pan until foaming, add the onions, cover and cook gently until softened, about 15 minutes. Add the ginger, cinnamon, cumin seeds, paprika and sultanas and continue to cook until the onions are starting to caramelise.
Place equal amounts of the stuffing mixture on top of the roasted squash, then return to the oven for 10-15 minutes to allow the juices to trickle through the squash. Serve immediately with some of the pan juices spooned over.