• Move the puff pastry into the refrigerator if it's frozen. Do this a day in advance. If possible, also roast the squash and prepare the duxelles the day before you plan on baking and serving the wellington.
  1 package puff pastry
• Start by roasting the butternut squash. Preheat the oven to 200°C (400°F). Halve the squash lengthwise and place cut-side down into a baking dish or on a baking sheet with high sides.
  1 medium butternut squash
• Roast until soft, about 40 minutes. You should be able to easily pierce it with a fork when it's ready. Set aside to cool fully, at least four hours. Once cool enough to handle, peel the half you want to use (keep the other half for something else) and cut the bulbous part off so that you just have the solid neck of the squash to use for the wellington. I usually cut both ends off to make it as neat as possible. Refrigerate after cooling if possible.

Duxelles

• Heat a large frying pan over medium heat. Once hot, add the oil.
  3 tablespoons olive oil
• Add the finely chopped mushrooms and onion, and cook for about ten minutes, or until reduced in size and fragrant. If it's catching at all, reduce the heat.
  400 grams (14 oz.) mushrooms, 1 medium onion
• Stir in the herbs, salt, and pepper. Add the wine and stir. Cook for another ten minutes, stirring occasionally. After this time the duxelles should hold its shape pretty well when pressed (it will be better once fully cooled).
  1 teaspoon herbes de Provence, 1 teaspoon sea salt, ½ teaspoon black pepper, 100 ml (⅖ cup) dry white wine
• Transfer the duxelles to a heat-safe bowl and cool to room temperature. Once cool, cover and transfer to the refrigerator to chill for at least four hours, preferably overnight.

Assembly

• All of the elements must be cold before assembling. Don't skip chilling.
• Roll out the thawed puff pastry and cut a piece about 24 x 12 cm (9 ½ x 4 ¾ in.). Reserve the remaining pastry.
• Place the pastry rectangle onto a parchment paper-lined baking sheet. Top with a layer of the chilled duxelles, using about two-thirds and reserving the remaining mushroom mixture to coat the squash** later. Be sure to leave a border of about 2cm (1 in.) around the mushrooms so that the pastry can be sealed later.
• Use the remaining duxelles to coat the squash, spreading with the back of a spoon. It should stick pretty well. If it's too tricky to get it to stick to the sides, simply spread another layer on the top instead.
• Top with the remaining puff pastry and seal the edges. You can crimp it as pictured, seal with a fork, or simply press to seal. Any pattern will largely disappear after baking.
• Place the wellington in the refrigerator and chill for at least 30 minutes, or up to 24 hours. If chilling for a long time, cover loosely with foil so that it doesn't dry out.

Baking

• Preheat the oven to 200°C (400°F). Score the top of the wellington with slits or any design that you'd like. Optionally brush with an egg wash (if it doesn't need to be vegan) to add a more golden colour.
  Egg wash
• Bake for 35-40 minutes, or until golden (dark golden with egg wash). If using fan-forced, simply bake on the middle rack for the whole baking time. If using conventional, place the wellington on the lower rack for the first ten minutes then move to the middle for the rest of the baking time.
• Cool for about ten minutes before slicing and serving. A very sharp knife will help to get cleaner slices but except a bit of flaking from the pastry.