Cover the bottom of the slow cooker pot with onion rings. Place chicken breast-side down onto the onions, cover it with salsa.
Cook on High about 5 hours. Make sure there is no redness at the bones and the juices and clear. An alternative way: insert a cooking thermometer into the thick of the thigh down to the bone – it should read 165°F/74°C.
Transfer chicken to a platter, cover with two layers of aluminum foil and let it rest in a warm place for 10 minutes. Cut up and serve.