Put the onions in a food processor and blitz to a paste.
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.