Add the onion, garlic, carrots and celery to a 6-quart slow cooker.
Place chicken breasts on top of the vegetables in the slow cooker. Add the Italian seasoning, bay leaf and pepper.
Pour 8 cups of water over the ingredients. Place the lid on the slow cooker and cook on high for 4 hours or low for 8 hours.
After cooking, carefully move the chicken to a cutting board and discard the bay leaf. Add the egg noodles to the slow cooker, stir to combine and replace the lid. Turn the heat to high if not already.
Use two forks to shred the cooked chicken breasts. Return shredded chicken to the slow cooker and stir to combine with the soup. Simmer for 8-10 minutes or until the noodles are tender.
Add salt to taste and stir in a handful of chopped parsley for garnish just before serving.