In a medium saucepan, cooke onion in butter over low heat until tender. Add flour, salt, and pepper, stirring until smooth and bubbly. Gradually stir in milk. Cook, stirring constantly, until mixture thickens. Stir in the sour cream. Add tuna, sherry, and parsley. Heat through, but do not boil. Spoon into a serving bowl or chafing dish and sprinkle with the slivered almonds, if desired. Serve with pastry shells or hot cooked rice. Serves 4.
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