Place the 7 oz poached chicken breast in a small pot and pour in 2 cups low-sodium chicken broth so the breast is mostly covered (use a pot size that allows this). Season the broth with kosher salt and pepper to taste.
Bring the broth to a gentle boil over medium-high heat, then reduce the heat to maintain a simmer.
Simmer the chicken for 5 minutes.
Remove the pot from heat, cover tightly, and let the chicken sit in the hot broth for 15–20 minutes, or until the thickest part of the breast registers 160°F on an instant-read thermometer.
Drain the chicken, let it cool until it is easy to handle, then cut it into small cubes.
While the chicken cools, prepare the produce: peel and cut 1 medium apple into small cubes; cut 1 cup red seedless grapes in half; chop 1/2 cup celery.
In a medium bowl, combine 1/4 cup light Hellmann's mayonnaise, 2 tbsp 0% Greek yogurt, and kosher salt and pepper to taste. Mix until smooth to make the dressing.
Add the cubed chicken, halved grapes, cubed apple, and chopped celery to the dressing. Stir gently until everything is evenly coated.
Chill the salad in the refrigerator until you are ready to eat.
Just before serving, stir in 2 tbsp pecans or walnuts.
Serve the chicken salad over 6 cups mixed baby greens (such as spinach and arugula).