Pour the oil into a medium skillet and heat over medium-low. Add the onion and celery, and saute them for a few minutes. This helps to bring out their flavor in the curry. (If you’re in a rush, you can skip this step and add the raw onion and celery to the slow cooker.)
Transfer the onions and celery to the slow cooker. Add the apples and raisins. Trim the fat off the chicken thighs if needed, then add the whole chicken thighs on top of the apples.
In a measuring cup, measure out the chicken broth. Add the curry powder, cumin, and cinnamon to the measuring cup, then stir the ingredients together. Pour the mixture over the chicken in the slow cooker.
Cook on low for 1 ½ to 2 hours, or until the chicken reaches an internal temperature of 165 degrees. Note: My Rival CrockPot tends to cook things fairly quickly, so your chicken might take longer. It’s safest to determine when the meat is done cooking by watching the internal temperature.
Carefully remove the chicken from the slow cooker. Let it cool down for a minute, and once it’s ready to handle, cut it into bite-sized pieces. Return the chicken to the slow cooker and stir everything together. Taste the mixture and add up to a half teaspoon of salt if needed. (This will depend on how salty your chicken broth is.)