3 cups 1/2-inch dice peeled celeriac (celery root; from about one 18-ounce celeriac)
1 12-ounce russet potato, peeled, cut into 1 1/2-inch chunks (about 2 1/2 cups)
1/4 cup cream or milk
2 tablespoons (1/4 stick) butter or olive oil
Cook celeriac in large saucepan of boiling salted water 15 minutes. Add potato and boil until celeriac and potato are very tender, about 15 minutes longer. Drain. Return to same saucepan; stir over medium-high heat until any excess liquid in pan evaporates, about 2 minutes. Add cream and butter; mash until mixture is almost smooth. Season to taste with salt and pepper.