From America's Test Kitchen Season 9: Resurrecting the Roast Beef Dinner
Root vegetables like carrots, parsnips, turnips, and celery root can add an earthy, intriguing flavor to mashed potatoes, but because root vegetables and potatoes have different starch levels and water content, treating them the same way creates a bad mash. We wanted a potato and root vegetable mash with a creamy consistency and a balanced flavor that highlights the natural earthiness of these humble root cellar favorites.
We found that a 1:3 ratio of root vegetables to potatoes provided an optimal consistency, although the root vegetable flavor was barely recognizable. Caramelizing the root vegetables first in a little butter helped bring out their natural earthy sweetness; this step also boosted the flavor of the overall dish. To use just one pot, we first sautéed the root vegetables in butter until caramelized and then added the potatoes with a little chicken broth. This gave us great flavor, but the mash had a gluey texture. The answer was to remove the starch from the potatoes by rinsing the peeled, sliced potatoes in several changes of water ahead of time.
Russet potatoes will yield a slightly fluffier, less creamy mash, but they can be used in place of the Yukon Gold potatoes if desired. Rinsing the potatoes in several changes of water reduces starch and prevents the mashed potatoes from becoming gluey. It is important to cut the potatoes and root vegetables into even-sized pieces so they cook at the same rate. This recipe can be doubled and cooked in a large Dutch oven. If doubling, increase the cooking time in step 2 to 40 minutes.