Take the cleaned prawns in a small heat resistant bowl. Drizzle 1/2 tsp of mustard oil. Place the bowl in a sauce pan or kadhai with little water and cover and cook in low heat for 8 minutes approximately. The cooking time will depend on the size of the prawns.
Alternately, you could directly place the prawns in a kadhai with a little bit of mustard oil and cover and cook over very low flame. The chances of the prawns overcooking here is high. So you have to keep a watch and it cooks in less than 5 minutes.
When the prawns change colour and have become soft, turn off the heat. Discard the water and take the prawns in a mixing bowl. Using your fingers, roughly break the prawns into small pieces.
Add the finely chopped onions. Point to be noted here is that the onions have to be really fine, tiny pieces.
Grate the garlic over a microplane or use a gralic press to add the fresh garlic to the bowl. Add chopped coriander leaves, chopped green chilies, 1 tbsp of grated coconut, salt and drizzle the rest of the mustard oil. Give them all a nice mash with your fingers.
Garnish with freshly grated coconut and serve.