Preparation

• Put the celeriac and potatoes in a pot, cover with water by at least an inch, and add 1 Tbs. of the salt. Bring to a boil. Reduce the heat to medium, cover partway, and cook until very tender, 20 to 25 min. Drain. Return the vegetables to the pot and set it over medium heat for 1 to 2 min., shaking and stirring to evaporate any excess water. Rinse a small saucepan in cold water (this will make the pan easier to clean later); pour in the milk (or half-and-half). Bring to just below a simmer over medium heat; set aside. Force the vegetables through a ricer or food mill and return them to the boiling pot. Beat in the butter with a wooden spoon. Add the milk to the potatoes a little at a time, beating vigorously after each addition. Stir in the horseradish and lemon juice. Add the remaining 1-1/2-tsp. salt, season with pepper, and serve.