Spread onion rings in the bottom of a slow cooker; place chicken on top, breast-side down. Pour salsa over chicken.
Cover slow cooker. Cook on High until no longer pink at the bone and the juices run clear, about 5 hours. An instant-read thermometer inserted into the thickest part of the thigh near but not touching the bone should read 165 degrees F (74 degrees C).
Transfer chicken to a cutting board, cover with a doubled sheet of aluminum foil, rest in a warm area for 10 minutes before cutting.
