Discard the head, and skin and flake the trout fillets. Puree the cream cheese, sour cream, and horseradish in a mini food processor. Add the fish and pulse to a make a smooth dip. Transfer to a bowl and stir in the onion and parsley, and the lemon juice, to taste. Refrigerate until firm, about 1 hour. Serve with celery or endive leaves.
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