Put half the onion, all the garlic, ginger and chilli flakes into a food processor with half the lemon juice, 1/2 tsp salt and 50 ml of water. Blitz to a puree.
Heat the oil in a large frying pan or wok over a moderate to high heat. Add the cumin seeds and mustard seeds and fry for 2-3 minutes until they begin to crackle and pop. Add the onion puree and cook for 5 minutes until it begins to stick and smell fragrant.
Add the remaining sliced onion and the peppers. Cover, reduce heat to moderate and cook for 15 minutes until the onions and peppers have softened and are starting to caramalize,
Add the tomatoes and chickpeas and cook for 2 minutes more, then check for seasoning. Add the garam masala and mix well, then add the rest of the lemon juice and sprinkle over the coriander.
Serve with the seasoned yoghurt, accompanied as preferred.