Fill a large stockpot with water and 2 T of salt. Bring to a boil. Place the chicken breasts in the boiling water and cook until the juices run clear or the meat thermometer reads 165 degrees. Remove the chicken from the pot and place in a large glass baking dish, spreading the chicken out to help it cool. Refrigerate chicken 4 hours or until completely cooled.

In a large bowl, combine onion, celery, walnuts, mayonnaise, salt and pepper. Mix with a large spoon. Cut cooled chicken into strips and add to bowl. Season with additional salt and white pepper to taste. Salad can be made up to one day in advance. Holding it any longer will bring out the unpleasant aftertaste of onion. Refrigerate until ready to serve. Split croissants horizontally and fill with chicken mixture.