Put the stock, lime leaves, fresh root ginger, chilli and lemon grass in a large saucepan. Cover and bring to the boil. Add the mushrooms and simmer for 2 minutes. Break the noodles into short lengths, drop into the soup and simmer for 3 minutes.
Add the baby spinach and prawns and simmer for 2 minutes until the prawns are heated through. Add the lemon juice. Remove and discard the lemon grass stalk and season the soup with black pepper before serving.
Notes & Wine Advice
For hot coconut soup, make up the soup as above, adding just 450 ml (¾ pint) stock and a 400 ml (14 fl oz) can coconut milk, plus 2 teaspoons ready-made Thai red curry paste. Continue as above and serve sprinkled with a little chopped coriander.