On a sheet pan lined with parchment paper, drizzle olive oil over chicken breasts. Sprinkle with a bit of salt and pepper. Roast at 375 degrees until done or the internal temperature reaches 165 degrees. this typically takes 30-40 minutes. Let cool. Remove the skin and bones. Tear or chop the chicken into small 1-2 inch pieces, so that the dressing will have little pockets to soak into. Set aside.
In a large bowl, combine chopped nuts, grapes, raisins, celery and onion. Mix well. Stir in chicken.
In add mayo- 1/4 cup at a time, all the yogurt, all the mustard, salt and pepper herbs and dry mustard to chicken mixture. Add in 1/4 cup of mayo and test to see if you need to add more. Add more mayo if needed- often the chicken can be very moist and it does not need a full cup of mayo. Let cool at least 1 hour. Overnight is best. Serve over a bed of fresh bibb lettuce.
Taste the salad for saltiness and if you have enough dressing. I do not like "wet" chicken salad, so I add my mayo in 1/4 cup portions at a time.