Combine the stock, mushrooms, bamboo shoots, ginger, garlic, soy sauce and chilli flakes in a saucepan. Bring to the boil, then reduce the heat to low, cover and simmer for about 15 minutes while you assemble the rest of the ingredients.
Mix the cornflour with the vinegar in a small bowl until smooth, then set aside.
Return the soup to a rolling boil. Drizzle in the beaten egg while stirring slowly to create long strands. Stir in the spring onions, coriander and cornflour mixture. Reduce the heat and simmer, stirring occcaisonally, for about 3 minutes until the stock has thickened slightly.
Stir in the tofu and cook briefly until heated through. Serve the soup garnished with shredded spring onions and red chilli slices.