Wash lemon grass, dab dry and cut in half first crosswise, then lengthwise. Tie pieces together with kitchen string. Peel ginger and onions. Dice or chop ginger finely. Cut onions into fine slices. Clean the chilli, cut lengthwise, wash and remove the seeds. Chop the pod finely.
Wash the mangetouts, drain, clean and cut them in half diagonally. Wash, clean, drain and cut the pak-choi into slices. Clean and trim the mushrooms and cut them into thin slices.
Heat the oil in a large pot, fry the ginger and onions in it. Stir in chilli and curry paste, pour in broth. Add lemon grass and coconut milk, bring to the boil and simmer for 2-3 minutes. Add mangetouts, pak choi and mushrooms, bring to the boil and simmer for another 5 minutes.
Cook rice noodles according to package instructions. Remove lemongrass from the soup. Season soup with salt and lime juice and add the drained noodles to the soup.