Place sliced onions in your slow cooker and set chicken on top. Zest the lemon and add the zest to the slow cooker. Then slice it in half, squeeze all the juice over the chicken, and throw the lemon halves in. Add the basil leaves and garlic. Pour the olive oil over the chicken and then add the salt. Cook on high 4-5 hours or low for 6-8 hours. After its done cooking, remove the meat and be sure to put all the skin and bones back in along with some other ingredients to make slow cooker chicken broth.