Spread the onion over the bottom of the Slow Cooker.
Place the chicken in front of you and remove all strings.
Cover the outside of the chicken with the spice mixture.
Place the lemon and left over spice mixture in the chicken cavity.
Carefully pick up the chicken and place it on top of the onions on the bottom of the slow cooker.
Turn the slow cooker on at the highest setting and leave it for at least 5 hours.
Check whether the chicken is cooked by measuring with a meat thermometer into the thick part of the chicken. This should indicate 75 degrees Celsius. Make sure you don't hit the bone when measuring the temperature of the chicken, because that heats up faster and can therefore already be 75 degrees, while the rest of the chicken is not yet.
Remove the chicken from the slow cooker when it reaches 75 degrees Celsius. You can eat it right now (but I'm positive you can't resist the tip below to make the skin crispy and crispy):
Turn on the oven grill and place the chicken including all juices in a roasting pan.
Slide it under the grill for 10 minutes for a delicious crispy crust.