Grind the onion pieces to a fine paste. Lastly add grated coconut, salt and red chilli powder to the onion mixture. Blenderise it again without adding water. Take it out. heat two spoons of oil in a pan for seasoning. And add mustard, cumin seeds & red chilli. Let it sputter. Add this popu/ tadka to the onion mixture and mix well. Garnish with coriander leaves. Serve with dosa, upma, pongal or idli..