How to Make It
In a large heavy-bottomed saucepan, combine the rice, sliced ginger and 12 cups of water and bring to a simmer. Cook over moderately low heat, stirring occasionally, until it starts to look creamy, about 30 minutes. Add the sweet potatoes and cook, stirring occasionally, until the rice and potatoes are tender and the porridge is thick, about 15 minutes longer. Season with salt.
Meanwhile, in a small bowl, combine the oil with the scallions and finely chopped ginger and season with salt.
Divide the congee between bowls and drizzle with the scallion-ginger oil.