In a large saucepan, heat the oil then add the onions and celery and season them with salt and pepper. Cook the two stirring occasionally until they soften (5-7 minutes). Transfer them to a very big bowl. I ended up using the bowl of my electric mixer.
Add a little more oil and turn the mushrooms and sage into the pan and season them with salt and pepper. Cook, stirring frequently until the mushrooms have browned (about 5 minutes). Pour in the wine and cook, stirring and scraping the bottom until the liquid has almost all evaporated.
Transfer it to the bowl with the celery and onion.
Add the walnuts and bread and then pour over enough stock to just wet the bread but not enough to make it soggy.
Stir the eggs through the stuffing mix.
Put 4 cups of stuffing aside for the turkey and spread the rest into the bottom of an oiled baking dish.
Store in the fridge for up to 1 day.
To cook the stuffing, preheat the oven to 185C(365F) and bake, uncovered until it is golden (about 30 minutes).