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Wash, peel and dice the vegetables into 1cm cubes. Tip the celeriac, swede and carrots (but not the sweet potato) into a microwave-proof container or large heatproof glass bowl, add the stock and cover with a microwaveable plate or lid. Heat on high for 10 mins. Carefully remove from the microwave and stir everything around. Cook for another 10 mins, until the swede is softening.
Stir through the sweet potato cubes, adding a splash of water if you need to. Cover and cook again on high for 10-15 mins until everything is soft. Drain off any of the water in the dish, reserving it, then add 20g butter to the veg. Leave to stand, covered, for 5 mins, before mashing with a potato masher to make a coarse mash – you don’t want it completely smooth. Add some of the reserved cooking liquid, if needed. Season.
Put the remaining 20g butter and the mace in a microwave-proof dish and heat for 1-2 mins until melted and foaming. Season, then stir in the walnuts. Pour this over the mash, scatter with the parsley and serve straightaway.