Finely chop celery and onion. Slit green chillies in the middle.
Heat oil in a pan, add mustard and cumin seeds. Once Mustard seeds start to splutter, add celery, onion and chilli and fry for not more than 2 minutes and let it cool.
Add seasoned celery and onion in the yogurt, salt and mix well. Garnish with corriander leaves and serve with any rice dish.
