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.