Wash the cabbage and slice it thinly.
Peel the onion, halve it and slice thinly into half-rings.  Deseed and finely slice the green chili.
Fry the onion in 1 tablespoon of oil until soft but not browned.  Add the mustard seeds and cook until they pop.
Add the chili and cabbage, with 1 tablespoon of water and salt.
Cover the pan and cook for about 6 minutes, or until the cabbage is just soft.  Serve hot.