All over the low carb world you'll see "Mashed Cauliflower" as the primary replacement for "Mashed Potatoes". That's fine ... and good ... and it totally works. Often times, people will toss a potato into the mix to add a little potato-y texture. Again, this is fine, but it will definitely boost the carbs.
I wanted to do something that would give it a little textural variety, but without using potatoes. Instead, I used "celery root" ... also known as "celeriac".
This is what the bulbs look like:

It's got a very neutral taste and a smooth consistency, when pureed. It's got a slight celery taste, but is very low carb and is an excellent companion to the cauliflower. HIGHLY recommended! I've also added a bit of garlic, because ... well ... I just like garlic. This would be excellent with fresh herbs, cheeses, bacon, etc. Anything you'd toss into your mashed potatoes ... you could toss into this and it would be wonderful. Yum!
Note: Here's a
quick video showing how to clean and peel celeriac. Once it's been peeled, cut it into chunks about the same size as your cauliflower florets. If they are the same size, they will cook evenly, at about the same amount of time.