2 pounds baking potatoes, peeled and cut into 1/2-inch dice
1 firm medium celery root (about 1 pound), peeled and cut into 1/2-inch dice
2 tablespoons unsalted butter
4 small leeks, white and light green parts, sliced crosswise 1/2 inch thick
1/2 cup heavy cream
Freshly ground pepper
1 cup buttermilk
1/4 teaspoon freshly grated nutmeg
In a large saucepan, cover the potatoes and celery root with water, add a pinch of salt and bring to a boil. Simmer over moderate heat until the vegetables are tender, 35 to 40 minutes. Drain the vegetables and return them to the pan.
Meanwhile, melt the butter in a small saucepan. Add the leeks, cover and cook over moderate heat for 5 minutes. Add the cream, season with salt and pepper and simmer, uncovered, until the leeks are tender, about 7 minutes. In another small saucepan, gently heat the buttermilk; do not boil or it will curdle.
Coarsely mash the potatoes and celery root. Stir in the hot buttermilk and then the creamed leeks. Season with salt, pepper and nutmeg and serve.