Marinate the prawns with a pinch of salt and turmeric powder for 5 minutes. Heat oil in a pan and temper with nigella seeds.
Saute for a few seconds and then add the marinated prawns. Stir fry till they change colour.
Now add the chopped bottle gourd, salt, turmeric powder and green chilies. Cover and cook on a low flame till done. If required, sprinkle some water at intervals.
When the bottle gourd is cooked and is soft, add the mustard paste, mustard oil and coriander leaves. Mix everything well and simmer for a minute.
Let it remain covered for 2-3 minutes and then serve with hot steamed rice or chapatis.