I love celeriac! It's so versatile as well as low carb, so it's absolutely perfect for this WOE - so it's really worth trying it.
Dauphinoise is good, as prettybird says - but my favourite is making a mash with it - but adding leeks. There's something about the texture of leek that gives the mash a texture that's pretty close to that of potato.
So this is how to make it:
Peel your celeriac and then cut it into small chunks. Boil, in salted water, until tender - this will take between 15 and 20 mins, depending on the size of the chunks, but you do want it properly soft.
While it's cooking, slice 2-3 leeks. If your celeriac is a large one make it 3.
Once you know the celeriac is cooked, add the sliced leeks to the pan and continue to boil until the leeks are soft - this will only take a couple of minutes. Then drain everything in a colander/sieve and leave this to steam dry for a minute or so. (There's quite a lot of moisture in leeks, so it's important to do this - you could also put them back into the pan over a low heat to dry it all out a bit).
Then, add a large knob of butter - at least 25g, more if you want! - and using a stick blender, blend it all. Be careful as you're doing this, it will mash very quickly and it's better with a little bit of texture rather than making it a puree.
Taste and season well with Maldon salt/sea salt and freshly ground black pepper.
It's lovely on its own but also fab as a topping for shepherd's pie.