Pour the chicken stock into a large saucepan with the coriander stalks, one of the chillies (halved lengthways and deseeded), the lemongrass and the ginger.
Bring to the boil and simmer gently for 10 minutes. Strain and discard the flavourings.
Return the stock to the pan, bring back to a simmer and add the chicken, spring onions and mushrooms. Simmer for 5 minutes until the chicken is cooked.
Add the green beans, broccoli, spinach, fish sauce, lime juice and tamarind paste, if using. Simmer for a few minutes. Stir in the tomatoes, coriander leaves and the remaining chilli, finely sliced.
Cook's tips: to get ahead, make up to the end of step 2 the day before; chill. This is also good made with prawns, in which case add the prawn shells to the base stock for the initial simmer, then discard at the end of step 2. The prawns will only take a few minutes to cook.