I make a "sort of" mashed potatoes with cauliflower and potato. I do use some white potato, either one small fresh potato or one servings worth from a box of mashed potato flakes or granules (the cheapest brands often have the fewest ingredients - generally only dehydrated potatoes and sometimes a bit of vegetable oil. The oil amount is too small to add fat calories, but prevents the dehydrated potato pieces from clumping together).
Mashing just cauliflower makes a pretty good imitation of mashed potato, but adding one serving of real potato (even instant) adds only about 100 calories for the whole batch, and the taste and texture comes MUCH closer to real mashed potatoes.
Using one large head of cauliflower and the one small potato or one serving of instant potato flakes makes AT LEAST 5 servings (I think I got 8 half cup servings out of my last batch) so each serving is only getting an additional 12 to 20 calories from the potato.
I use chicken stock (one can or water and soup base or a bouillon cube) about 2 cups. Then put in the whole trimmed cauliflower head (the water doesn't cover the cauliflower) and cover the pan and simmer until the cauliflower is tender. I drain out any of the broth into a cup (in case I need extra moisture). I microwave the potato in the mean while (if I'm not using flakes). Then I mash the cauliflower with a potato masher (I mash the potato with it, or add the flakes as I mash) with a little bit of skim milk (maybe a quarter cup) and a little bit of the reserved broth. Sometimes I toss in green onion (the white and an inch or two of the greens), sliced thin right before draining (they're also good tossed in raw during the mashing instead).
Personally, I like them better than the typical mashed potatoes. They have a lighter, creamier texture and just melt in your mouth - Almost a cross between colcannon (and if you add the onions) champ, two irish mashed potato dishes (recipe link below).
http://fall-recipes.suite101.com/art..._champ_recipes