Cook celery root in large pot of boiling salted water 5 minutes. Add potatoes to pot; cook until all vegetables are tender, about 20 minutes. Drain. Return vegetables to pot; stir over medium-high heat until dry, 1 to 2 minutes. Remove from heat; add butter. Using potato masher, mash vegetables until butter is incorporated. Add 1/2 cup milk; mash until almost smooth, adding more milk as needed. Stir in celery leaves. Season with salt and pepper.
Recipe courtesy of Epicurious