2 boneless, skinless chicken breasts (about 1 pound total)
1 small yellow onion, halved and peeled
Coarse salt and ground pepper
1/4 cup chopped toasted walnuts
2 tablespoons golden raisins
1 celery stalk, finely chopped
1/2 small red onion, finely chopped
3 tablespoons mayonnaise
1/4 cup nonfat plain Greek yogurt
2 teaspoons chopped fresh tarragon leaves
Bread and lettuce leaves, for serving