Place the chicken breasts in a pan and cover with broth. Bring to a simmer and cook until done, about 8-10 minutes. Allow to cool in the broth, then cut into small dice.
In a small bowl mix together the mayonnaise, sour cream, orange zest, and juice. Add the dressing to the chicken.
Stir in the apple and celery.
Place a leaf of lettuce on each piece of bread. Top with chicken salad.