Celeriac doesn’t merely look unappetising in the shop: it looks like some kind of strange alien pod, that in due course will hatch a monster, or crawl away under its own steam. Shame, as it’s rather tasty.
Peel and chop equal amounts of celeriac and potato. The celeriac is “peeled” by dint of hacking off the outside with a large knife, until all the dirt, tentacles and other hideous bits have gone, and you’re left with something that looks like a giant lump of parsnip flesh. Both should be chopped into pieces roughly an inch across, I think.
Put them in a pan and pour over just enough cold water to cover, add some salt and bring to the boil. Once they’re boiling, reduce the heat, and give them about 15 minutes. After that, test with a skewer: you don’t wash them to turn into slush, but you want them soft enough that you’ll be able to do battle with a potato masher and not emerge red faced and defeated.
Drain, and mash. I added milk and crème fraîche, because that was what was on hand.