Step 1Place the chicken breasts in a 5-quart saucepot along with the broth, ginger, onion, garlic, green onion and lemon grass. Bring to a boil. Reduce the heat, cover and simmer until the chicken is tender, 30 to 45 minutes. Remove the chicken from the broth, let it cool, then cover and refrigerate it overnight. Cover the chicken broth and refrigerate it overnight as well.
Step 2The next day, skim any excess fat from the top of the soup. Heat the soup until it's hot, then strain it through a cheesecloth-lined strainer.
Step 3Return the strained broth to the pot and bring to a simmer. Add the rice. Return the soup to a simmer, cover and cook until the rice is tender, 15 to 20 minutes.
Step 4Add the mushrooms and bok choy during the last 5 minutes of cooking. Remove the skin and bones from the chicken breasts and dice the meat. Place the chicken in a microwave safe bowl or container and heat in the microwave 1 to 2 minutes to warm.
Step 5Place the chicken into the bottom of a serving bowl and ladle the soup over the top.