Remove the giblets from the chicken, rinse it inside and out with water, and pat dry with paper towels.
Rub the chicken inside and out with salt, pepper, and bouillon powder. Generously season the chicken with homemade Creole seasoning. Stuff it with 3 garlic cloves and the lemon slices, and then refrigerate until ready to cook. You can do this up to 24 hours ahead.
Remove chicken from the fridge when ready to cook. Put the sliced onions, the rest of the garlic, and bay leaves at the bottom of the slow cooker.
Set the chicken on top of the onions, breast side up.
Tuck the potatoes, celery, and carrots into the available space around the chicken. Then add the thyme.
Cover the slow cooker, set it to high, and cook the chicken for 2½-3½ hours. Or cook it for 4-5 hours on low. The exact cooking time will depend on the size and type of chicken you are using. The internal temperature should register 165℉ (75℃) in the thickest part of the breast or 175-180℉ (80-85℃) in the thickest part of the thigh.
To crisp up the skin, if desired, carefully transfer the chicken to a roasting pan and broil it in the oven for about 5 minutes. Remove the chicken from the oven and let it stand for 10 minutes before carving, allowing the juices to settle back into the meat.
Discard the bay leaves, smashed garlic, and lemon, and enjoy.