Place chicken and water in a large pot over high heat and bring to a boil. Reduce heat, cover and simmer, skimming fat as needed, 30 minutes.
Place the halved onion, bay leaves, peppercorns, whole celery, whole carrots and lemon grass in the pot and simmer, covered 1 hour.
Strain broth and reserve chicken. When chicken is cool enough to handle, remove skin and cut meat into bite-size pieces.
Return strained stock to pot over high heat, and stir in chicken base, chopped celery and chopped carrots. Bring to a boil, then reduce heat, cover and simmer 20 minutes, or until carrots are tender.
Chop celery leaves and stir into pot with the noodles. Simmer until noodles are cooked, about 10 minutes more. Stir in chicken and heat through.
