A broth made from the celery root trimmings replaces half of the cream usually found in potato gratins without loss of flavor or texture. Celery root has a haunting flavor that always reminds me of truffles, which are an excellent addition should you be so lucky. (If I were using truffles, I would use all cream in the dish.)

Serves 4 to 6

1 garlic clove and butter for the dish
1 celery root, about 1 pound, scrubbed
1 pound potatoes, preferably Yellow Finn or Yukon Gold
½ cup cream
2 teaspoons Dijon mustard
Salt and freshly milled pepper
1 cup grated Gruyère

Preheat the oven to 375°F. Rub a 2-quart gratin dish with the garlic and then with butter.

Peel the celery root and put the parings in a 3-quart saucepan with 3 cups water and whatever remains of the garlic. Set a steamer over the top and bring to a boil. Quarter the root, then slice it ¼-inch thick. Steam for 5 minutes and remove to a large bowl.

Peel the potatoes, slice them into thin rounds, and steam for 5 minutes or until tender, then add them to the celery root. Strain the cooking liquid, measure 1¼ cups, and mix it with the cream and mustard. Pour it over the vegetables and toss well. Season with ¾ teaspoon salt and pepper to taste. Transfer the vegetables to the gratin dish, smooth them out, and cover with the cheese. Bake until bubbling and browned on top, about 30 minutes.