Put the celeriac into a saucepan, cover it with cold water and a teaspoon of salt.
Bring it to the boil and simmer for 15-20 minutes.
Drain it into a colander, then return to the pan on a low heat for 1 minute to allow any excess water to evaporate.
Blend the celeriac in a food-processor until it is smooth.
Return it to a clean pan, stir in the cream- and butter