Adapted from Vegetarian Cooking for Everyone by Deborah Madison
1 garlic clove and butter for the dish
1 celery root, scrubbed well
1 pound potatoes
½ cup cream
2 teaspoons Dijon mustard
Salt and freshly milled pepper
1 cup grated Gruyere or Swiss cheese
Preheat oven to 375. Rub a 2-quart gratin dish with the garlic and then the butter.
Peel the celery root and put the pairings in a 3-quart saucepan with 3 cups of water and whatever remains of the garlic. Set a steamer over the top and bring to a boil. Reduce heat medium-high. Quarter the celery root, and then slice it ½ inch thick. Steam (make sure the steamer is covered) for 5 minutes and remove to a large bowl.
Peel the potatoes or scrub well, 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 1 ¼ 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.