Put the shitake mushroom and dried fungus strips in a bowl, cover with warm (not boiling) water and set aside for 20-30 minutes. Rinse & drain, finely slice & set aside. Slice the cooked meat, bamboo shoots and carrot into thin strips.
Heat a wok or soup pot over a medium heat. Add the garlic & ginger paste and warm through for 30 seconds. Add the chicken stock, salt, msg, sugar, light soy sauce, dark soy sauce & hot bean sauce and mix well.
Add the sliced meat and vegetables to the simmering soup. Bring to the boil & simmer for 2-3 minutes. Add the white pepper & black vinegar and mix well.
Add the potato starch and water mix, stirring well until the soup is slightly thickened (you won't need all of the mix). Whisk the egg thoroughly, then drop it slowly into the simmering soup. Let the egg sit for a few seconds, then stir gently to create ribbons throughout the soup. Add the spring onion, ladle the hot & sour soup into bowls and serve.