large (600 g) chicken legs • olive oil • large white onion, peeled and thinly sliced • coriander seeds, crushed • tamarind pods, shelled and seeds removed or 2 tbsp tamarind paste • tomato purée • chicken stock • medium tomatoes, quartered • pak choi, trimmed and halved • freshly ground sea salt • freshly ground black pepper • fresh coriander, roughly chopped