Put the onions in a food processor and blitz to a paste. (Ma would have used a stone grinder).
Heat the oil in a heavy-based pan over a medium–high heat. Add half of the onion paste and cook until it turns light brown. Add the remaining onion paste, ginger, garlic, turmeric, chilli powder, cumin, ground coriander, green chillies, salt and 4 tablespoons of water and bring to the boil. Reduce the heat, cover and simmer for 15 minutes.
Remove the lid, increase the heat and cook the paste until you can see the oil coming to the surface. Add the prawns and cook, uncovered, over a medium heat for 10–15 minutes until the prawns have changed colour and are cooked through.
Serve immediately, garnished with coriander
Note: If you are using mustard oil you need to heat it to smoking point to reduce the pungency of the oil. Please ensure your kitchen is well ventilated so the oil does not set off the smoke alarm. Leave the oil to cool slightly before starting the recipe.