How to Make It
Place the potatoes and celery root in a large pot and add enough cold water to cover. Bring to a boil and add 2 teaspoons salt. Reduce heat and simmer until the vegetables are very tender, 18 to 20 minutes. Drain and return the vegetables to the pot.
Add the half-and-half, butter, 1 teaspoon salt, and ¼ teaspoon pepper and mash to the desired consistency. Sprinkle with the chives.