You may have had céleri rémoulade in France- it's that ubiquitous (but delicious) salad dish you'll find in charcuteries up and down the land; frankly, not dissimilar to the American coleslaw. The "celeri" bit is shredded celery root. The "remoulade" is a mustard-flavoured mayonnaise or dressing.
Celery root is also called celeriac. They're those knobbly, bulbous, earthy things which look a bit like large turnips. Making céleri rémoulade is straightforward enough:
Take a large, sharp knife and slice off the brown, knobbly skin. You'll find that the celeriac turns yellow with exposure to air, so you will need to work relatively quickly. Cut the celeriac into large chunks and then shred it in your food processor. I've finally mastered the art of working mine, and like to shred things very thinly, so that you end up with delicate, tiny strands. It's very satisfying for some reason.
Dump the shredded celeriac into a bowl, and immediately mix in some fresh lemon juice and sea salt. This will stop the celeriac from turning yellow, in effect, keeping it white in colour.
Now for a bit of conjecture. How to make the rémoulade dressing? Some recipes call for a simple mayonnaise, seasoned with salt and pepper, and flavoured with a large dollop of Dijon mustard. And that's what I used the last time I made céleri rémoulade (adding a tablespoon of boiling water to the mayonnaise to lighten the colour and give it a lift). But to be completely honest, even then I found my home-made mayonnaise to be too rich, too thick, even slightly cloying in taste, swamping the subtle taste of the celeriac.
In his excellent blog, Living the life in Saint-Aignan, Ken Broadhurst suggests that to achieve that genuinely authentic charcuterie taste, half the dressing should be made from the mustard-mayonnaise and the other half from crème fraiche. This might well be the solution.
I've also noted that Julia Child used an eggless mayonnaise, by slowly dripping boiling water into a dollop of warm mustard (so that it forms an emulsion), and then adding the oil (in the usual way) bit by bit, finishing it off with a tablespoon or so of white vinegar, and a seasoning of white pepper and salt. This eggless "mayonnaise" is then mixed in with the shredded celeriac, with chopped parsley and sour cream added to taste. I like the idea of this version.
By the way, I found that the céleri rémoulade improves if left over-night to marinate in the 'fridge. The lemon, salt, and mustard helps to "cook" the celeriac and removes that raw taste.