Heat the stock and simmer the scallions for about 3 minutes. In a slow, thin stream, add the egg to the gently simmering stock and gently stir so that threads of egg form. Serve immediately.  Variations: Use a vegetarian broth. Add one thinly sliced shiitake mushroom per serving. Add seaweed, re-hydrated according to the package directions. Add 4 tablespoons diced tofu. Flavor it with tamari or sesame oil.   (Recipe♦389)