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Brush a 2-litre bundt tin with olive oil, getting into all the crevices. Reserve 50g of the walnuts, then blitz the rest to fine crumbs in a food processor. Mix with the flour, then use to coat the tin, tipping any excess into a large bowl. Set the bowl aside. Arrange the reserved nuts in the base of the tin. Peel the garlic, then blitz with the parsnips and carrots in the food processor until finely chopped. Set aside.
Heat 2 tbsp olive oil and the butter in a large, deep frying pan over a medium-high heat and fry the mushrooms for 10 mins until browned. Stir in the onions and fry for about 15 mins until slightly softened and caramelised. Tip in the blitzed veg mixture and stock, turn up the heat to high and cook for 5 mins until everything has softened and the liquid has evaporated. Add the thyme leaves, spices and miso. Season to taste, then spoon the mixture onto a large plate, spread out and leave to cool slightly. Once slightly cool to the touch, heat the oven to 200C/180C fan/gas 6.
Meanwhile, blitz the sourdough to crumbs in a food processor, then tip into the bowl with the excess flour and walnut mixture. Blitz the chestnuts until finely chopped, then tip into the bowl with the breadcrumbs and season well.
Add the cooled veg mixture to the bowl and mix everything together well. Stir in the eggs and lemon zest until evenly combined. Tip the mixture into the prepared tin and pack down using the back of a spoon. Cover with foil and bake for 30 mins, then uncover, turn up the temperature to 220C/200C fan/gas 7 and bake for a further 30 mins until dark brown. Gently release the edges of the roast from the tin using a cutlery or palette knife. Put a serving plate on top of the tin, then flip over to invert the roast onto the plate. Leave to cool with the tin on top for 10-15 mins before removing.
Meanwhile, make the gravy. Heat the oil and butter over a medium-high heat in the frying pan you used to cook the veg mixture. Fry the onions and bashed garlic with a large pinch of salt for 15-20 mins until the onions are caramelised and slightly jammy. Tip in the flour, stir to combine and cook for a few minutes. Slowly whisk in the marsala and miso, and cook for a few more minutes to reduce slightly before pouring in the stock. Cook for 5-10 mins more until the gravy has reduced and thickened. Season to taste and remove the bashed garlic cloves. Will keep for frozen for up to three months. Serve alongside the parsnip roast.