Heat the oven to 200C (180C fan)/390F/gas 6. Cut the squash at the point where the neck broadens out, and trim the stalk off the top. Peel the neck, then cut it in half lengthways. Warm the butter or oil, then whisk in the Marmite (or soy sauce) until well combined. Put the squash on a baking tray, brush all over with the glaze, then roast for about 35-40 minutes, until tender. Remove and leave to cool.

While the squash is roasting, for the mushroom layer melt the butter or oil in a frying pan over a medium heat, then sauté the shallots, stirring, until softened. Add the mushrooms, season and cook, stirring regularly, for about 10 minutes, until soft and dry; add the thyme leaves towards the end of this time. Stir in the walnuts and cranberries, followed by the breadcrumbs and the reserved squash glaze, take off the heat and leave to cool.

Bring a large pan of generously salted water to a boil, then drop in the spinach and blanch until just wilted. Drain, run through with cold water, then squeeze out as much liquid as possible.

Heat the butter or oil in a frying pan over a medium heat, then add the garlic and cook, stirring, until fragrant. Stir in the drained, blanched spinach, fry, stirring, until it stops steaming, then leave to cool.

Put the pastry on a piece of greaseproof paper, roll out into a roughly 30cm x 40cm rectangle, then cut into two equal rectangles. Scoop most of the duxelles on to one half, leaving a 1cm border clear all around the edge, then arrange the spinach down the middle of the duxelles mix. Neatly lay the two pieces of squash on top of the spinach, then spoon a line of duxelles on top of the squash.

Roll out the second piece of pastry a little wider than the first, paint the edges with egg wash or milk, then lay it on top of the squash and press down on to the bottom piece of pastry to seal. Transfer the wellington to a lightly greased baking tray.

Heat the oven again to 200C (180C fan)/390F/gas 6. Brush the wellington all over with egg wash or plant milk, then bake for 40-50 minutes, until golden brown. Leave to cool slightly before slicing and serving.