A fluffy mashed or pureed vegetable side dish is always a crowd-pleaser, and the humble potato is often the star. This recipe gives the spud a rest and turns to the season’s bounty of root vegetables for inspiration. Nutrient-rich roots are storehouses of energy and guaranteed to add an extra dimension of color and sweet, earthy flavor to the popular mash. Most roots, such as rutabaga, parsnip, carrot, and celery root, are delicious when mashed or pureed. They can be used individually or combined for more nuance and balance in flavor. This recipe combines parsnips with mellow celery root, along with a dollop of roasted garlic. The celery root tamps down the parsnips’ sweetness, and the garlic adds buttery richness to the dish. This is an unfailingly flexible recipe, and you can tinker with the ingredients and method to your taste and preference. For instance, there are several ways to …