Make the seasoning mix: In a small bowl, mix salt, paprika, garlic powder, onion powder, ground black pepper, thyme leaves, and minced rosemary. Set aside.
Pat dry the chicken. Rub the chicken with about 3 tablespoons of olive oil. Then, rub the seasoning mix all over.
In a large 6 or 7-quart slow cooker, arrange the onions, carrots, and parsnips. Season with a few drizzles of olive oi, salt and pepper to taste, and toss to combine. Place the seasoned chicken on top. If desired, tie the legs with kitchen twine. Note: There is no need to add broth, the chicken will release plenty of juices.
Cover the slow cooker and cook on HIGH for 3.5 to 4 hours, or until tender and cooked through. The center should reach at least 165 degrees F.
Once ready, transfer the chicken onto a large sheet pan (be careful since it will be very tender). Broil in the oven for 3 to 4 minutes until the skin is browned and slightly crispy. Remove from the oven and let it rest for 10 minutes.
To serve, carve or slice the chicken. Serve with the vegetables and the slow cooker juices (or make a gravy, see notes below). I like to serve it over mashed potatoes. Enjoy!