* if soup is not thick enough, add 2 more spoons of cornstarch (mixed into cold water) into the soup Prepare Ahead: 1. In a small bowl, cover the mushrooms with 1/2 cup of warm water and let them soak for 30 minutes. Discard the water. With a cleaver or knife, cut away and discard the tough stems of the mushrooms, and shred the caps by placing one at a time on a chopping board. Cut them horizontally into paper-thin slices, and then into thin strips. 2. Drain the pieces of bamboo shoots and bean curd, and rinse them in cold water. Shred them as fine as the mushrooms. 3. With a cleaver or sharp knife, trim the pork of all fat. Then shred it, too, by slicing the meat as thin as possible and cutting the slices into narrow strips about 1-1/2 to 2 inches long. 4. Have the above ingredients, stock, salt, soy sauce, pepper, vinegar, cornstarch mixture, egg, sesame seed oil and scallions within easy reach. To Cook: Combine in a heavy 3-quart saucepan the stock, salt, soy sauce, mushrooms, bamboo shoots and pork. Bring to a boil over high heat, then immediately reduce the heat to low, cover the pan and simmer for 3 minutes. Drop in the bean curd and the pepper and the vinegar. Bring to a boil again. Give the cornstarch mixture a stir to recombine it and pour it into the soup. Stir for a few seconds until the soup thickens, then slowly pour in the beaten egg, stirring gently all the while. Remove the soup from the heat and ladle it into a tureen or serving bowl. Stir in the sesame seed oil and sprinkle the top with scallions. Serve at once.