Chicken Salad with Grapes and Walnuts with Maple-Dijon Dressing

This is an excellent potluck recipe. Last time I took this somewhere, grownup, mature people were fighting over the leftovers. This will serve 15-20 people in a potluck-type situation.

6 - 7 large skin-on, bone-in chicken breasts
  (single breasts)
1 onion, cut into chunks
1 celery stalk, cut into a few pieces
1 teaspoon whole black peppercorns
1 bay leaf

1 cup halved black seedless grapes
1 cup halved green seedless grapes
4 stalks celery, cut lengthwise, then thinly sliced
  (make sure it's fresh, crunchy, and green)
1 bunch green onions, thinly sliced
1 cup walnuts, lightly toasted and broken up
half a bunch of flat-leaf Italian parsley, minced

1/2 cup olive oil (regular, not extra-virgin)
1/4 cup real maple syrup
2 Tablespoons dijon mustard
Rice vinegar, a few generous shakes

Freshly ground black pepper
Kosher salt

Put chicken, onion, celery, peppercorns, and bay leaf into a stockpot and cover generously with water. Bring to a gentle simmer, then immediately turn the heat down to low. You do not want it bubbling, or the chicken will get tough and dry. After awhile, take out a piece of the chicken and cut down into it to see if it's cooked through. If not, return it to the pot. As soon as the chicken is cooked through, remove it from the pot (you can strain the stock and save it to make soup if you don't want it to go to waste).

When the chicken is cool, remove the skin and bones and cut it into chunks.

To toast the walnuts, put them on a baking sheet and into a 300 degree oven. Check every couple of minutes until they just start to turn golden. Remove immediately and put on a plate to cool.

In a large bowl, combine the chicken, walnuts, green and black grapes, celery, green onions, and parsley. In a jar, shake up to combine the dressing ingredients: olive oil, maple syrup, dijon mustard, and rice vinegar. Taste and see whether the mixture is to your liking. If you like it sweeter, add more syrup, more acidic, add more vinegar. Pour dressing over the chicken mixture and toss thoroughly. Add plenty of freshly-ground black pepper and kosher salt to taste and toss again.