In a medium pot, cover the rice with the water. Bring to a boil over high heat. Reduce the heat to low and simmer until the rice is tender and porridgelike, about 25 minutes.
Meanwhile, cut the piece of ginger into slivers, then thinly slice crosswise. Stack the slices, removing the bottom rounded slice, then cut the slices lengthwise into thin slivers.
Add the chicken stock, fish sauce, salt, and sugar to the rice, bring to a simmer and continue cooking over low heat for 10 minutes. Stir in half of the ginger and simmer for 10 minutes longer. Turn off the heat.
Ladle the soup into shallow bowls. Garnish with the shredded chicken, sliced scallions, chopped cilantro, and the remaining slivered ginger.
