DAIKON RADISH
with
I don't know that I'd consider daikon a good substitute for potatoes, but I have had them roasted before, and they're great that way. I typically see them pickled. I also like roasted parsnips, which are more like carrots.
I used a variation of this recipe: http://sarahscucinabella.com/2013/05/29/roasted-daikon-radish-carrots-and-peppers/
I've also had some really good whipped cauliflower, and those recipes are generally a really good mashed potato substitute in my opinion.
Enjoy!