Place your chicken breasts in a cold 3 quart pot of cold water. Bring to a boil, than turn off the heat and let cool. If time is a problem boil the chicken 15 to 20 min. or until done.
Chop up your celery, apple, and quarter your onion and set aside. Cover and refrigerate if you are waiting for your chicken to cool or wait until you are ready to put the salad together
When the chicken has cooled, cut into pieces to place into the food processor. Add your onion, mayo, yogurt and salt and pepper. Pulse ingredients to well mixed.
Place mixed ingredients into a large bowl. Add the rest your ingredients and mix well by hand. If too stiff, add more mayo but not too much. Refrigerate 1 hour. Taste, salt and pepper if needed and serve.