Wash, pat dry and keep the prawns/ shrimp aside.
In a small pan, dry roast the coriander seeds, cumin and black peppercorns for 2 minutes only until lightly fragnant. Do not burn. Cool and keep aside.
In a blender, add all the ingredients for the wet paste/ masala and grind to a smooth paste. Feel free to add a tbsp of extra water to make a smooth paste.
Heat a heavy bottomed pan and add the oil. Once warm, add the chopped onions and saute them over medium heat until they are light golden brown.
Add the chopped tomatoes and let them cook and soften for about 5 minites.
Keep the flame low and add the ground wet (masala) paste. Cook this for 5 minutes after which stir in the mix of coconut milk and water. Bring this mixture to a boil.
Add the prawns and let them simmer. At this stage, you may add in 1-2 slit green chillies depending upon your heat tolerance.
After the prawns have simmered on low to medium flame for about 7-9 minutes, mix in 1-2 tsp jaggery powder and add salt to taste.
Switch off the flame after 2 minutes and mix in chopped coriander leaves.
Serve hot with steamed rice.