A broth made from the celery-root trimmings replaces half of the cream that’s 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.)

Steps

1. Preheat the oven to 375 degrees. Rub a 2-quart gratin dish with the garlic, then with the butter.
2. 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.
3. Quarter the root, then slice it ¼-inch thick. Steam for 5 minutes and remove the roots to a large bowl.
4. Peel the potatoes, slice them into thin rounds, and steam for 5 minutes or until tender, then add them to the celery root.
5. Strain the cooking liquid. Measure 1¼ cups liquid and mix it with the cream and mustard. Pour it over the vegetables and toss well. Season with ¾ teaspoon salt and pepper.
6. 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.