Prepare the filling: in a medium skillet heat the oil over low heat.
Add the onion, salt, pepper and half the sage and cook, stirring, for 5
minutes. Add the chestnuts and cook for another 4 minutes, stirring.
Remove from the heat and add the panko or breadcrumbs and the apple.
Add the remaining sage and taste for seasoning, adding more salt and
pepper as needed.

Preheat the oven to 400 degrees.

Place the squash in a medium sized baking or gratin dish. Season the
squash with a generous amount of salt and pepper. Divide the stuffing
between the two cavities, pushing down and mounding it. If you have
extra stuffing you can bake it in a small skillet or baking dish
alongside the squash. Dot the top of the filling with 1 tablespoon of
the butter cubes and pour 1 cup of the cider on top of the stuffing and
squash, letting it drip down the sides and onto the bottom of the pan.

Bake for 30 minutes. Add the remaining butter on top of the stuffing
and pour the remaining half cup cider on top. Bake another 30 minutes, or
until the squash feels tender when tested with a small, sharp knife or
the tines of a fork. Serve hot or room temperature with the sage leaves
as a garnish. You can serve them whole or cut in half.