In a 2- to 3-quart pan, combine rice and 2 3/4 cups water. Bring to a boil over high heat and cook until most of the water is absorbed, 7 to 10 minutes. Turn heat to low, cover, and cook until rice is tender to bite, 10 to 15 minutes longer.
In 3- to 4-quart pan, bring soy sauce, fresh ginger, sugar, five spice, and 1 3/4 cups water to a boil over high heat.
Meanwhile, slice pork into strips about 1/8 inch thick and 2 inches long. Slice potatoes 1/4 inch thick.
When soy mixture is boiling, stir in pork and potatoes. Return to a boil, then cover and simmer over low heat, stirring occasionally, until potatoes are tender when pierced, 15 to 18 minutes. Stir in onions.
Mix cornstarch with 2 tablespoons water. Add to potato mixture and stir until sauce boils. Ladle into soup bowls and serve with rice.