Bring chicken stock, galangal, lime leaves, lemon grass and hot chilli paste to the boil in a large pot. Stir in fish sauce, lime juice, shallots, tomato, mushrooms, pak choi and carrot. Reduce heat to medium-low and simmer until the vegetables are tender, about 25 minutes. Garnish with coriander and spring onions before serving.
If your local supermarket doesn't stock lemon grass, galangal or kaffir lime leaves, you can find them in Chinese/Oriental speciality shops.