Heat a large saucepan, spray the bottom with oil, add the ginger, garlic and chillies and stir-fry for 1 minute. Add 1.8 litres (3 pints) boiling water and bring to a simmer. Stir in the miso paste, lime juice and noodles and cook for 1 minute. Cover and set aside.
Heat a large wok or frying pan, spray it with oil and stir-fry the chicken, mushrooms and sweetcorn for 2—3 minutes. Add the peas and cook for 2 minutes.
Transfer the soup to 4 bowls, spoon over the vegetables and chicken and top with watercress. Serve the soy sauce on the side.