Preparation: 20 minutes. Cooking: 30 minutes.
Preheat oven to 450 degrees. Peel and grate the celery root. Heat the cream in a large saucepan over moderately high heat. Add celery and season with salt and pepper. Bring to a boil, reduce heat to moderate and simmer until softened, about 10 minutes.
Empty into a large mixing bowl. Add the eggs, one at a time, stirring well after each addition. Use the butter to grease a 9x13 inch baking dish or 1 1/2 quart capacity gratin dish. (Alternatively, butter 6 individual 1 cup gratin dishes.)
Fill about 3/4 full with the celery mixture. Cover with Parmesan and bake until browned and set, about 20 minutes. Serve hot. Serves 6.
