Rinse and drain the prawns. Pat them dry and put them in a non-reactive bowl. Add the yogurt, chillies and salt. Mix well, cover and marinate overnight in the fridge.
When ready to cook the curry, pour the oil into a karhai, wok or heavy-based pan, about 23cm in diameter, and set it over a medium heat.
Spoon in the cumin seeds, swirl and brown for 10 seconds. Add the onions. Sauté for about 10 minutes, or until brown all over.
Add the garlic, reduce the heat to low, stir and fry for two minutes. Mix in the turmeric and stir for one minute.
Now add the green chillies, increase the heat to medium and stir for one minute. Mix in one-and-a-half teaspoons of the garam masala and stir for one minute.
Add half the salt and all of the tomatoes and chilli flakes. Cook for two minutes, then add 120ml/4fl oz of boiling water. Combine to make a thick sauce. Simmer for three minutes, then add the remaining salt.
Stir in the prawns and their marinade and cook over a medium heat until they are just opaque and cooked through. Sprinkle the remaining garam masala over the top and stir. Fold in the chopped coriander and serve.