Add the potatoes to a pan of boiling water.
Simmer for 10 minutes, then add the celeriac.
Simmer for a further 10-15 minutes until the potatoes and celeriac are tender.
Drain in a colander and return to the pan, shake over medium heat for a few seconds to evaporate any remaining water.
Take the pan from the heat and mash the potato and celeriac with a potato masher until smooth.
Return to the heat; make a well in the centre by pushing the mixture to the sides of the pan. Add the cream and the butter; heat for approximately 30 seconds then thoroughly beat the mixture together.
Serve piled in a heated dish, garnished if preferred with finely chopped celery and celery leaves.

Heat the oil in a large heavy based pan. Fry the onion until soft.
Add the celeriac and carrots and stir fry for about 5 minutes.
Add the stock and seasoning - mix well.
Cover the pan with a tight fitting lid and bring the soup slowly to the boil.
Reduce the heat and simmer for about 20minutes.
Tip the soup into a blender and blend until smooth.
Serve and enjoy!