Cut the butternut squash in half lengthwise. Scoop out and discard the seeds, then pierce the skin all over with a sharp knife. Season the flesh with a little salt then microwave, in batches, on full power for 10-12 minutes, or until the flesh is just tender, but not cooked all the way, when pierced with a knife.
Scoop out the flesh in the centre of the squash halves, leaving a small border all around. You can use the squash flesh in the stuffing or set aside to use in soup.
Heat the olive oil in a large casserole dish. Fry the onion, garlic, leeks and celery for 7 minutes until softened.
Add the mushrooms and continue to cook for 3 minutes.
Add the chopped chestnuts, rice, stock, seasoning and cranberries and stir together.
Bring to a simmer, cover and cook for 20-30 minutes, stirring occasionally, until the rice is cooked. Some wild rice needs a lot longer to cook, so check the package instructions. Top up with a little extra water or stock if the pan is too dry.
Stir in the herbs and taste – adjust the seasoning, if needed. If you are using the squash flesh, add it to the filling now.
Preheat the oven to 180C and line a large tray with baking paper.
Pack in as much of the filling into both sides of the two squash and sandwich together. Tie together with kitchen string or wrap tightly with 2-3 layers of foil.
Place on the tray and roast for 30-40 minutes until the flesh is tender all the way through when pierced with a knife.
Make the gravy. Melt the vegan spread over low heat and fry the onion until softened and slightly caramelised, 10-12 minutes, stirring occasionally.
Stir in the flour and gradually add the stock. Stir in the soy sauce or marmite and continue to simmer until the gravy is slightly thickened.
Slice the squash and serve with the gravy.