Put the mushrooms in a bowl, cover with cold water, and soak for at least 20 minutes, or until softened. Remove the mushrooms, squeeze dry, and discard the stems. Cut the mushrooms into thin strips and mix with 1 teaspoon oil.
Meanwhile, combine the pork, soy sauce, and 1 teaspoon sugar and marinate for 10 minutes. Mix in the cornstarch (cornflour). Mix in 1 teaspoon oil just prior to cooking.
Cut the preserved mustard hearts into thin strips. Soak in a bowl of cold water for 5 minutes, then drain.
Heat the remaining 2 tablespoons oil in a wok or large skillet/frying pan over high heat, add the pork, and stir-fry for about 1 minute. Add the mushrooms and stir-fry for another minute. Add the preserved mustard hearts, chile, remaining 1 teaspoon sugar, and 1 tablespoon water and sauté for another minute until the pork is cooked through.
Transfer to a serving plate and serve.