Heat oil in a large frying pan or wok over medium high heat. Stir in mustard seeds. When mustard seeds begin to pop, stir in onion. Cook for 1 minute, then stir in green capsicums, turmeric, salt and chilli powder. Reduce heat to medium low, cover, and cook 3 to 4 minutes. Stir in chickpea flour, mixing well; cover, and cook 2 minutes more.