Rinse the chicken and pat it dry. Rub the entire chicken with olive oil, then generously sprinkle with salt and pepper.
Place the dry stuffing mix in a bowl and add the chicken stock. Stir to mix and combine well. Place the stuffing inside the chicken cavity.
Line the bottom of the slow cooker with the halved potatoes (try and get one single layer of potatoes for even cooking) and the onion slices. If desired, add in the crushed garlic for a deeper flavor. Sprinkle with salt and pepper. Place the chicken on top of the potatoes and onions.
Place the lid on the slow cooker and set on Low for 8 - 9 hours, or on High for 4 - 5 hours, until the chicken is completely cooked through and the potatoes are fork-tender.
Remove the chicken from the slow cooker at the end of cooking time, cover with foil, and let rest for 10 minutes before removing stuffing and placing in another bowl. Carve chicken as you normally would.
Pro tip: Don't want to cook a whole chicken? You don't have to! This same dinner can be made by placing chicken breasts on top of the potatoes and onions, and placing the stuffing on top of the chicken. Remember to reduce the cooking time by about half.
Pro tip: Placing the chicken upside down (breast-side down) on top of the potatoes causes many of the juices from the legs to run down into the breasts while the chicken is cooking. This is a French technique that I've tried in the oven, but it works so much better in the slow cooker!