Put the garlic, tamarind paste and ¾ of the chilli into a large saucepan and cook over a low heat for 2 minutes. Add the vegetable stock and slowly bring to the boil.
Add the soy sauce, fish sauce, sugar and lemongrass and simmer for 5 minutes. Add the red pepper and cook for a further 5 minutes.
Add the prawns, pak choi and noodles and simmer for 10 minutes, until piping hot throughout. Season with the lime juice and sprinkle over the spring onions and remaining chilli. Remove the lemongrass stalk before serving.