Don't be fooled into thinking that this is diet food though. This is a dish to which cream is added in order to make it lighter (!!) and easier to digest (!!!) It is tremendously popular in France, but I don't think many people bother to make it themselves. Most people buy it from their trusted local charcutière traiteur (delicatessen). If you do make it at home, it's not just a matter of grating some celeriac and coating it with mayonnaise. There's a trick to getting the deli style céleri rémoulade, as Ken explains in his blog. David Lebovitz has a recipe and discusses the tricks to making it on his blog too.
Make a mayonnaise by putting a whole egg and an egg yolk in a jug. Add a dollop of prepared mustard and some salt. Using a hand held blender, whiz the egg and mustard together, then start blending in a thin drizzle of olive oil until you have about two thirds of a cup of mayo. Then switch oils and add some sunflower oil in the same manner, until you have about a cup and a half of mayo. Next tip in a pot of thick cream (I use the 40% fat raw cultured cream I buy from my laitière) and whiz that around until it is all blended in.
Céleri rémoulade (at the back) as part of a mixed plate of salads.