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Boil the potatoes for 20-25 mins until tender, adding the celeriac after 5 mins. Drain and press through a potato ricer back into the pan once the surface water has evaporated, or mash well with a potato masher in the dry pan.
While the potatoes cook, melt the 140g butter over a medium heat, then sweat the spring onions for about 5 mins until soft and silky, stirring occasionally. Add the cream and season with salt, pepper and nutmeg. Bring to a simmer, then cook for another few mins until this thickens a little.
Stir two-thirds of the creamy onions into the potato and celeriac purée. Season to taste. If making in advance, cover the surface of the mash and the sauce with cling film, then set aside. Can now be frozen or made up to a day ahead. Just before serving, gently reheat the mash and reserved sauce. Spoon the mash into a serving bowl, pour the sauce on top and turn it over a few times to loosely mix. Make a well in the centre and drop in the remaining knob of butter.