Chop the tomatoes into small chunks.
Chop the onion small.
Chop the ginger, garlic, chillies very small.
Prepare the coconut milk by mixing 2 heaped tablespoons of powdered coconut milk in 400ml of hot water. If using canned coconut milk, use one heaped teaspoonful.
Heat the oil in a saucepan.
Add the onion and cook till they are soft.
Add the garlic, ginger, turmeric, chillies, tomatoes, lemon grass, cinnamon to the saucepan.
Strip the leaves off the curry leaf sprig and add them to the pan.
Let this fry for a minute or so, moving the mixture gently with the spoon.
Add the coconut milk.
Add a pinch of salt.
Bring the gravy to a simmer and let it simmer for five minutes.
Add the prawns and cook till they a just firm (15 to 20 minutes depending on the size).
Squeeze the lime over the dish just before serving.