Peal and remove the husk of ridge gourd and cut into small pieces.
Put a deep bottomed pan on the stove. Add oil and heat. Splutter mustard seeds. Add the onion paste and fry for a few minutes. Add chopped onion and fry until the contents turn brown.
Add enough water, cover the pan with a lid and allow to simmer in medium flame until the prawns and ridge gourd pieces are cooked well and the oil gets separated.
Top the preparation with finely chopped coriander leaves and serve hot with rice or chappathis.