Mashed turnip. Sounds horrible, but tastes better than potato I reckon. Bit of butter and milk, salt and pepper. One large turnip has about 5g of carbs and is a similar size as a portion of mashed potato that contains about 30g.
Sure, I'm no expert masher though! I cut it into 2 cm cubes (as much as you can get a round object into cubes) boiled it in salted water until I could get a fork into it (but not so that it falls apart), drained it, mashed it with a masher, put some milk and butter in there and finished the mashing with a fork on the side of the pan.
I didn't manage to get it as smooth as potato though, but it tastes much better I think.
Love mashed turnip and carrots, I like mine with a knob of butter and freshly ground black pepper, if it's too bland for some then try adding a teaspoon or two of Horseradish sauce before mashing.