Cream ¼ cup butter with ground mustard seed and grated mustard root. Coat bottom and sides of a shallow 2-quart ceramic or glass casserole dish with the mustard flavored butter.
Mix potatoes and celery root in a bowl and place mixture in baking dish. Level evenly over bottom and tamp down slightly.
Beat egg yolks and whole eggs with a fork to blend. Add heavy cream, salt and pepper to eggs and mix well. Pour egg mixture over potato mixture. Dot top with with remaining 2 tablespoons of butter. Bake in the middle of a 425° oven for approx. 30 minutes or until golden brown.
Turn oven down to 350°, remove casserole and loosely cover with foil. Return casserole to oven and bake another 12 to 20 minutes or until vegetables are tender when pierce with tip of a knife and the knife comes away clean indicating the custard is set. Allow finished casserole to stand for 10 to 15 minutes before serving.
Nutritional Analysis per serving: 768 calories, 12 g protein, 54 g fat, (63% calories from fat), 64g carbohydrates, 338 mg cholesterol, 6 g fiber, 946 mg sodium.