In a large pan, melt the tbsp of butter. Add the onions and
cook for 5 mins until softened, but not coloured. Add
the potato, celeriac, the litre of stock, 4 sage leaves and 2 strips of lemon peel. Bring
to the boil and simmer gently for about 30 mins, until
the celeriac is tender and the potato is collapsing.
Remove and discard the lemon zest and sage
leaves. Stir so the potato thickens the soup a little.
(At this point you can cool and keep covered in the
fridge for up to two days or freeze for up to 3 months.)
When you’re ready to eat, reheat the soup to just
simmering. Stir in the 200g of cheese. Serve with crisp sage
leaves for a special touch.