Celeriac undoubtedly makes one of the most superb Winter soups. In parts of France, such as the Savoie where cheese plays an important part in the cuisine, the soup is frequently topped with croûtes of toasted cheese.
Celeriac is that ugly looking bulb, hard to peel because of the lumps and bumps, but makes the most smooth soups.
Croûtes are pieces or slices of bread which have been grilled with cheese on top. These are floated on top of the soup. These are optional, of course, but delicious. Make them thick or thin, use Gruyere, or Parmesan or whatever cheese you have at hand.