Deshell, de-vein and clean the prawns carefully, keeping the head and tails intact.
Rub the prawns with a little turmeric powder and salt. Heat 1 tbsp of mustard oil in a frying pan till smoking hot. Cool the oil. Add the prawns and fry till light golden (be careful not to over fry the prawns). Keep aside.
Fry potato cubes till golden brown and tender. Keep aside.
Reheat the same oil, add the chopped onions, green chillies and fry till the onions are soft. Add the remaining turmeric powder, red chilli powder and salt.
Cook over a moderate flame till oil has been released from the masala. Splash a little water if needed, making sure that the masala does not stick to the pan.
At this stage, add the fried potatoes and stir in the posto paste. Continue cooking for 4-5 mins over a moderate flame.
Add 1/2 cup of warm water, give it a nice stir and bring to a gentle simmer.
Add the fried prawns, adjust the salt and cook for a further 3-4 mins till posto gravy thickens.
Finish with a generous drizzle of mustard oil and chopped coriander leaves. Serve hot with steamed rice.