Cut the celeriac into thin strips, or grate it on the coarse side of the grater. Mix all the rest of the ingredients together and stir thoroughly into the celeriac. Put in a cool place for several hours before serving so that the flavours can amalgamate and the celeriac can soften in the sauce.
Classic French simplicity at its best, this celeriac salad is wonderful winter food. Serve it with fresh granary bread, either as a starter or for a light lunch with a tossed salad.