To make the broth, char the onion and ginger over an open flame on the hob until they have started to blister and blacken.
Put the cinnamon stick, star anise and coriander seeds in a large stock pot and toast over a medium heat for 2-3 minutes until the spices release their aroma. Add the burnt onion and ginger to the pan, followed by the stock, dried mushrooms and soy sauce. Bring to the boil, cover with a lid and then leave to simmer gently for 1 hour. Add the lime juice and keep warm.
Meanwhile, heat the oil in a medium frying pan and cook the tofu for 5 minutes, turning until it’s browned on all sides. Remove and set aside. Add the mushrooms to the pan and stir fry for 2-3 minutes until soft. Remove and set aside.
Put the noodles in a large heatproof bowl and cover with boiling water. Leave for 3 minutes, until cooked, then drain.
Divide the noodles between four bowls. Divide the tofu, mushrooms, pak choi, spring onions and chilli evenly between the bowls, and then ladle over the hot stock. Garnish with lime wedges and a drizzle of sriracha for extra heat.