Place the miso powder or paste in a large saucepan with the measurement water, star anise, fish sauce, soy sauce, sugar and chilli, if using. Bring to the boil, then reduce the heat and simmer gently for 15 minutes.
Stir in the baby corn and mangetout and cook for a further 3 minutes or until almost tender. Remove the pan from the heat, stir in the chicken, tofu and mushrooms and cover to retain the heat.
Cook the noodles in a large saucepan of boiling water for 3–4 minutes, or according to the packet instructions, until tender. Drain well and divide between deep bowls, then ladle over the hot soup and serve immediately, garnished with the spring onion, if liked.