Rub spice mixture all over the chicken, including the back of the chicken, the sides and in the cavity.
Stuff the chicken cavity with lemon halves, sliced garlic and onions.
Place chicken, breast side down, in the slow cooker.
Cover and cook on low setting for 6-8 hours (or until juices run clear).
Squeeze some of that lemon juice inside the chicken before placing the lemons inside. Don't bother peeling the garlic or onions, put them in there as they are. No other liquid needs to be added during cooking. The chicken will produce plenty of juices while it is cooking. When slow cooking whole chickens, the skin will stay soft. We usually just remove the skin and serve it up that way with a little bit of gravy. But if you'd like to crisp up the skin, you can easily do that. After your chicken is finished cooking, remove most of the juices (don't toss that, it makes for delicious stock!), flip the chicken over so the breast side is up. Be careful, this chicken is SO tender! Turn the broiler to low in your oven. If you have an oven-safe slow cooker insert, you can just put the whole thing under the broiler for about 4-6 minutes (or until golden and crispy.) Keep an eye on it . If you are unsure if your slow cooker insert is oven safe, just take the chicken out of the slow cooker and put it in an oven safe pan.