- 1 pound celery root (celeriac), peeled, cut into 1/2-inch pieces (about 4 cups)
- 2 1/2 pounds russet potatoes, peeled, cut into 1-inch pieces (about 6 1/2 cups) [German Butterballs are a fine sub]
- 1/4 cup (1/2 stick) unsalted butter, cut into pieces
- 1/2 cup (or more) whole milk
- 1/4 cup chopped celery leaves
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.