The potatoes provide volume and a familiar flavor base, the parsnip adds sweet, creamy, earthy goodness. Potatoes are on the HHFM list as a good source of vegetable based protein. The celeriac sends the mash OVER-THE-TOP with hints of herbaceousness and celery, along with a hearty texture.
After putting down deep roots in the South, the sweet potato spread across America to almost every Thanksgiving dinner. Why "save" them for Thanksgiving? They're too good to "save" just for special occasions. Not only do they have more taste than bland white potatoes, they have more nutrition, more fiber and fewer calories.