Thickly slice jerusalem artichokes, and cook in salted water until soft. Drain. Make a bechamel sauce using the butter, flour and milk. Thicken for 15 minutes, mixing occasionally. Add the goat cheese, mix until melted and blended in the sauce. Remove from heat, add the egg yolk, and season to taste with nutmeg, salt and pepper. Preheat oven to 400 F (200 C). Lightly oil a small oven proof baking pan, and coat with bread crumbs. Arrange the jerusalem artichoke slices in one layer. Spread the goat cheese mixture on the jerusalem artichoke layer. Sprinkle additional bread crumbs, and bake 15 minutes or until golden.