Soak the mushrooms in warm water for 20 minutes. Drain, reserving the liquid.
Combine the reserved mushroom liquid and the stock in a saucepan and bring to the boil. Add the salt, soy sauce, pork, bamboo shoots and mushrooms. Cover and simmer 5 minutes.
Add the tofu and return to a simmer. Stir in the cornflour mixture and simmer 2 more minutes.
Beat the egg and slowly stir it into the hot soup. Add the vinegar and pepper. Serve, sprinkled with spring onion and sesame oil.