To prepare a celeriac you need to wash it and peel it like a potato. Cut it up into cubes and you can boil, roast or mash it! Celeriac will discolour very easily so if you're serving it raw, seal it with lemon juice or vinegar first. If there is any discolouring, it won't affect the taste. Celeriac will last quite a while if kept properly, and like other vegetables it will keep the longest if it is kept in a cool place.