Marinate the prawns with ½ tsp turmeric powder, salt and fish sauce. Keep aside for 15 minutes.
Pound the shallots, garlic, ginger and softened dried chillies in a mortar and pestle to a coarse paste. Do use the grinder if you must but strongly recommend the mortar and pestle.
Heat oil in a pan and add the coarse paste. Sauté on low to medium heat till all the water is absorbed and the oil starts to clear around the edges.
Add the chilli paste and turmeric powder; continue to sauté on low heat.
Once the onion mixture has caramelized well, add the tomatoes.
Add 1/4th cup water and cook for about 10 minutes or till the tomatoes are softened.
Then add the prawns and cook on high till done.
Finally, garnish with chopped coriander leaves and serve warm.