Place potatoes in the bottom of a slow cooker. Add onion over the top of the potatoes.
Remove the chicken parts inside the cavity and pat the outside of the chicken dry. Slice deep, garlic clove-width slits into your chicken with a knife. Make 4 cuts on each side of the chicken, and 1 in each of the drumsticks. Stick a sliver of garlic deep into each cut. Place the remaining chunks of garlic into the cavity.
Starting on the back of the chicken, season generously with about 1/2 of the rosemary and cracked salt and pepper. Add some seasoning inside the chicken's cavity as well. Flip the chicken and place it breast side up in the slow cooker on top of the potatoes and onions. Season the top of the chicken generously with the remaining rosemary and more cracked salt and pepper.
Cook on Low until the chicken is cooked through completely and the juices run clear when the chicken is cut, 8 to 10 hours, depending on the size. Serve with the potatoes and onions from the pot.