Cooking chicken breasts slowly in simmering water ensures they'll be tender, not rubbery. Onion and salt in the water add flavor without fat. If you have them on hand, you can also toss in a bay leaf, lemon peel, or whole peppercorns.
In a large straight-sided skillet or pot, cover chicken and onion with water by 1/2 inch; season generously with salt. Bring to a boil over high; cover, reduce heat to medium-low, and cook 5 minutes. Remove skillet from heat and let stand, covered, until chicken is cooked through (an instant-read thermometer should read 160 degrees), 12 to 14 minutes. Remove chicken from water and let cool completely. (To store, refrigerate in an airtight container, up to 3 days.)
Shred chicken, then mix with walnuts, raisins, celery, red onion, mayonnaise, yogurt, and tarragon. Make sandwiches with bread and lettuce.
Poached chicken is endlessly versatile. Try it tucked into tacos, stirred into soups, or tossed into salads.