Place chicken in the bottom of the slow cooker. Add onion, carrots, celery, garlic, thyme, bay leaf, salt, pepper, and chicken stock.
Cover and cook on low for 6-8 hours or on high for 3-4 hours, until chicken is fully cooked and vegetables are tender.
Remove and discard the bay leaf and thyme stems. Thyme leaves should have infused into the soup.
Transfer chicken to a bowl or cutting board, let rest for 5 minutes, then shred into small pieces and return to the soup.
If the slow cooker is set to low, switch to high. Add egg noodles, cover, and cook for 10 minutes or until noodles are tender.
Adjust salt and pepper to taste. Serve garnished with fresh parsley, if desired.