Pre-heat the oven to 200°C/gas mark 6. Add the 325g flour, the walnuts and salt to a medium bowl. Combine and transfer to the fridge to keep cold. Spread out the butternut and Brussels sprouts on a baking tray and drizzle 1 tbsp of the oil over the top. Toss with the thyme leaves and plenty of seasoning. Roast for 25 minutes, taking out the sprouts halfway through. Transfer to a large bowl and cover until needed. Preheat a baking tray or sheet.
While the vegetables roast, transfer the chilled flour mixture and vegetable spread cubes to a food processor and pulse briefly until the mixture starts to look sandy with small and large chunks.
Drizzle 4 tbsp of ice-cold water over the flour and butter mixture and pulse until just coming together. If the mixture looks too dry, add more water 1 tbsp at a time, pulsing briefly after each addition until it’s just the right consistency.
Turn the mixture out on to a lightly floured work surface and bring together with your hands, trying not to knead or overwork the dough. Cut into two pieces and wrap in clingfilm, flattening with the palm of your hand into a disc. Transfer to your fridge and leave to chill for 30 minutes.
Meanwhile, add the 450ml almond milk, onion, bay leaves, peppercorns and thyme sprig to a medium saucepan. Bring to a steady simmer and then turn off the heat. Cover and leave to infuse for 30 minutes.
Heat 1 tbsp of the olive oil in a large frying pan and add the mushrooms and garlic, sautéing for 5-6 minutes. Add the sage in the last minute, stirring through. Once cooked, add to the large bowl of roasted vegetables along with the chestnuts and pine nuts.
Add the remaining olive oil to a large pan and heat over a medium heat until hot and starting to shimmer on the surface. Add the 2 tbsp flour and fry in the oil for 2 minutes, continuously stirring until combined. Strain the almond milk into a jug and gradually add to the flour mixture, whisking well after each addition. Once all the milk has been added, add the nutmeg and continue to cook for a further 2-3 minutes, whisking often, until the sauce is thick enough to coat the back of a spoon well – it will continue to thicken as it cools. Leave to cool.
While the béchamel cools, use some of the vegetable spread to lightly grease the base and sides of a loose-bottomed 20cm cake tin (the sides should be 4cm in height). Lay out some baking paper and dust very lightly with flour – rolling the pastry dough out on this will help prevent it from sticking. Taking one pastry half, roll it out to 3mm in thickness and cut into a 28cm round. Picking up carefully on the rolling pin, roll it onto the cake tin and push it well into the edges, letting it evenly overhang on the sides. Prick the base a few times with a fork.
Combine the sauce with the vegetable mixture, then pour into the lined cake tin. Roll out the remaining pastry to 3mm thick and cut into a 28cm round. Dampen the edges of the pastry lining the tin and then drape the other pastry on top. Pinch the base and the lid together where they overlap in the tin, then trim away the excess pastry, leaving a 1½cm overhang.
Fold in the overhanging pastry, creating a scalloped edge with your thumb and forefinger. Gently roll out the pastry scraps and cut out some leaves and small balls of dough to decorate the top of the pie. Brush the top of the pie with the remaining almond milk. Cut 4 small slits in the top of the pie to allow steam to escape while cooking.
Transfer the pie to the oven on the hot baking tray and bake for 45 minutes. When the pie is golden brown on top, remove from the oven and leave to stand for 15 minutes. Serve.