Poached-Chicken-Salad Sandwiches

Name: Poached-Chicken-Salad Sandwiches
Prep time: 15 min
Total time: 15 min
Cuisine: American
Ingredients: 2 boneless, skinless chicken breasts (about 1 pound total); 1 small yellow onion, halved and peeled; Coarse salt and ground pepper; 0.25 cup chopped toasted walnuts; 2 tablespoons golden raisins; 1 celery stalk, finely chopped; 0.5 small red onion, finely chopped; 3 tablespoons mayonnaise; 0.25 cup nonfat plain Greek yogurt; 2 teaspoons chopped fresh tarragon leaves; Bread and lettuce leaves, for serving