Preheat the slow cooker if necessary; see the manufacturer's instructions. Put the chicken carcass into the slow cooker pot, breaking it into 2 pieces if necessary to make it fit. Add the onion, carrots, celery and bouquet garni.
Pour over the boiling water and add a little salt and pepper. Cover with the lid and cook on high for 5–7 hours.
Strain the soup into a large sieve, then quickly pour the hot soup back into the slow cooker pot. Take any meat off the carcass and add to the pot. Taste and adjust the seasoning if needed. Add the pasta and cook on high for 20–30 minutes or until the pasta is just cooked. Sprinkle with parsley and ladle into deep bowls. Serve with warm bread, if liked.