Line the bottom of the tin with an overlapping layer of squash (this will be your top when you turn it out, so make it pretty). Divide the rest of the squash into 2 and add half of it, tucking some of the sliced sage and onion mix between. Add half the stuffing and pack it down lightly. Add all the sliced potato, layering it with more onion and sage mix. Repeat the layering with the rest of the squash and stuffing mix. Cover with foil, put on a baking tray and bake for 2 hours, or until tender. Leave to sit for 10 minutes before turning it out.