If using nuts, add them to a small skillet. Toast the nuts over a medium heat until they smell fragrant, about 3 minutes. Be careful, they burn quickly! Cool completely.
Add yogurt, mayo, vinegar, dijon, honey and salt to a large bowl. Mix until combined. Season to taste with salt and pepper.
Add chicken, red onion, celery, scallion, grapes, tarragon, cooled nuts and remaining salt to the dressing. Toss to combine. Season to taste with salt and pepper. Serve on choice of bread.