Place chicken in a skillet, and sprinkle with lemon juice and 1/2 the rosemary. Pour broth into the skillet. Bring to a boil, reduce heat to low, and simmer 25 minutes, or until chicken juices run clear. Drain, cool, and dice chicken.
In a large bowl, toss chicken with lemon zest, remaining rosemary, walnuts, celery, blue cheese, mayonnaise, brown mustard, salt and pepper, and garlic powder. Chill in the refrigerator at least 1 hour before serving.