To a 6-quart slow cooker, add onion, celery, and carrots. Top with chicken, sprinkle the garlic, thyme, rosemary, and season with salt and pepper.
Pour the chicken stock all over the chicken, give everything a stir, and cover with a lid. Cook on LOW for 5-6 hours or on HIGH for 2-3 hours.
Taste and adjust the salt and pepper if needed, then return the chicken back in along with chopped parsley. Give everything a gentle stir and serve immediately.