In a food processor, cover and process chicken until finely chopped. Transfer to a large bowl; add the walnuts, celery and onion.
In a small bowl, combine the mayonnaise, sour cream, curry powder and Cajun seasoning. Pour over chicken mixture; toss to coat. Cover and refrigerate for at least 1 hour. Place lettuce and about 3/4 cup chicken mixture on each croissant.