Celery Root and Potato Gratin

Heat the oven to 375 degrees F. Rub a 2-qt. gratin dish with the smashed garlic, reserving what is left. Coat the dish with butter.

Peel the celery root; reserve the trimmings. Cut the celery root into quarters and then into 1/4-inch slices. Put the trimmings and the remaining garlic in a 3-qt. saucepan and add 3 cups water and the salt. Set a steamer over the top of the liquid and bring the liquid to a boil. Steam the celery root just until tender, about 5 min., and then transfer it to a large bowl. Steam the potatoes over the same liquid just until tender, about 5 min. Toss the potatoes with the celery root. Strain the steaming liquid and reserve.

Combine 3/4 cup of the reserved steaming liquid with the cream and mustard. Pour this over the vegetables, add the pepper, and toss. Pour the vegetables with the liquid into the gratin dish, smooth the top, and sprinkle with the cheese. Bake until the gratin is bubbling and browned on top, about 30 min.