In a bowl, combine the egg whites, sherry, sesame oil, and cornstarch, and whisk until thickened. Add the chicken, toss lightly to coat, cover, and refrigerate 3 to 4 hours, or overnight.
Drain the mushrooms, remove and discard the stems, and cut into strips. Set aside with the peas.
In a large pot, combine the soup base ingredients and heat until mixture boils. Add the chicken, mushrooms, and peas. Bring back to a boil and cook about 3 minutes, stirring to separate the chicken pieces, and skimming the surface to remove any impurities.
Meanwhile, dissolve the cornstarch in the water. Add to the soup, stirring constantly, and cook until soup thickens and turns glossy, 2 to 3 minutes. Remove from the heat, and taste for seasoning. Slowly add the beaten egg to the soup in a thin stream around the edge, stirring once or twice so that the cooked egg forms long threads. Serve warm in individual bowls, and garnish with scallions.
c.1998, M.S. Milliken & S. Feniger, all rights reserved