Bring a large pan of water to the boil. Halve the butternut squash lengthwise and remove the seeds with a spoon. Cook the squash in boiling water for about 25 minutes until tender. Leave to cool for 15 minutes.
Preheat the oven to 180 °C.
Finely chop the shallot. Thinly slice the shiitake and chestnut mushrooms.
Heat the olive oil in a large pan. Fry the shallot on a medium heat for about 3 minutes until translucent and soft. Add the mushrooms and thyme. Fry on a high heat for 5 minutes. Add the spinach in handfuls and let the spinach wilt. Season with salt and pepper. Leave to cool slightly.
Beat the egg and mix with the cooled vegetables.
Scoop some of the flesh out of the squash with a spoon. Stuff one half of the squash with the spinach mixture. Place the other half on top. Tie the kitchen twine around the squash to secure it and place in an oven dish.
Bake the butternut squash in the middle of the oven for approx. 1 hours until completely cooked through. Leave to rest for 10 minutes before serving.
