How to Make It
In a large pot, combine the chicken wings with 4 quarts of the water and the ginger, scallions and 2 tablespoons of the fish sauce. Bring to a boil over high heat and skim. Cover partially and simmer over low heat until the stock has reduced by half, about 3 hours. Skim occasionally during cooking.
Strain the stock into a large saucepan and discard the solids. Add the asparagus and crabmeat to the stock and bring to a boil over high heat. Cover partially, then simmer over low heat until the asparagus is tender and the flavors come together, about 5 minutes.
Pour the egg whites into the soup in a slow steady stream and blend a few times so the whites cook into strands. In a small bowl, stir the cornstarch with the remaining 3 tablespoons of water until smooth, then stir into the soup. Simmer until the soup thickens, about 1 minute. Season with the remaining 1 tablespoon of fish sauce and salt and pepper. Ladle the soup into bowls, garnish generously with cilantro and serve hot.