Cook spaghetti according to package directions, drain and put into buttered bowl. Meanwhile, melt butter over low heat, blend in flour, salt, nutmeg, and stir constantly until mixture is smooth and bubbling. Remove from heat, stirn in broth and cream, put back on heat, and bring to a boil, stirring constantly until thickened. Stir in wine and cheese. Add sauce to spaghetti. Add tuna, green pepper, mushrooms, and egg yolk to spaghetti mixture and blend throroughly. Pour into buttered baking dish and bake uncovered at 350F for approximately 30 minutes. Sprinkle parsley over top before serving.