In France,  it is often served as a purée enriched with cream, which is delicious as a vegetable side dish. In fact, it is so popular you can buy frozen purée that you just warm through in a saucepan. Cook the celeriac in boiling water in small chunks, as it does take some time to become tender. Try mixing cooked potato and celeriac together for a new take on mashed potatoes. Don't forget the butter!