Heat the oil over a medium heat in a wok or large frying pan. Add the mustard seeds and, when they begin to pop, fry for 10 seconds. Add the crushed spices and fry for a minute. Add the curry leaves and, as they splutter, add the onion. Fry for 7-8 minutes, stirring often. Add the marinated chicken and turn the heat up slightly, frying for 4 minutes. Season with a little salt.