Put the tofu on a plate lined with kitchen paper and allow to stand for 10 minutes to drain.
Heat the oil in a wok until hot and fry the tofu for 2–3 minutes until golden brown, stirring frequently.
Meanwhile, soak the noodles in boiling water for 2 minutes, then drain.
Pour the stock into a large saucepan. Add the ginger, garlic, lime leaves and lemon grass and bring to the boil. Reduce the heat, add the tofu, noodles, spinach or pak choi, bean sprouts and chillies and heat through. Add the coriander and fish sauce, then pour into deep bowls. Serve with lime wedges and chilli sauce.