In a large saucepan, combine the broth, onion, egg white, and crushed shell and simmer the mixture for 10 minutes. Let mixture stand for 20 minutes, then strain through a sieve lined with a double thickness of cheesecloth. In a small heatproof bowl, let the gelatin soften in the Sherry for 5 minutes. Heat the mixture over a pan of simmering water, stirring, until the gelatin is dissolved, and add it to the broth. Let the aspic cool, but do not let it set up. Rinse a 2-qt. mold with cold water, cover the bottom with a thin layer of the semi-liquid aspic. Chill for 3 hours or until it is firm. Pour the remaining aspic into a shallow baking pan and chill it. Cut the hard cooked egg white and truffles into decorative shapes. Garnish the edges of the bottom of the mold with these. In a small heatproof bowl, let the gelatin soften in the Sherry for 5 minutes and heat the mixture over a pan of simmering water, stirring, until the gelatin is dissolved. In a saucepan, cook the scallion in the butter over moderate heat, stirring, until softened. Do not brown the onion. Stir in the broth, tomato paste, and gelatin mixture, and simmer, stirring, for 1 minute. Remove the pan from the heat and stir in the ham. Season the mixture with nutmeg, salt, and pepper, and let it cool. In a small bowl, beat the cream to stiff peaks and fold it into the ham mousse. Chill, set over a bowl of ice water, stirring occasionally, for 10 hours or until it begins to set. Pour the mousse into the mold, spreading it evenly, and chill it, covered for 3 hours or until set. Unmold the mousse onto a chilled platter and garnish with the remaining chilled aspic, chopped, and parsley. a 1968 Gourmet Mag. favorite.