Heat a heavy bottomed kadai and dry roast all the ingredients on a low heat till golden colour / fragrant for 20 - 25 minutes.
Grind gashi masala ingredients in a mixer without adding water. Grind till the oil leaves from the sides of the jar. It should be a very fine paste. Store in an airtight container for further use or store in refrigerator.
Heat oil in a non-stick pan / kadai. Fry sliced small onions till pink colour and fragrant.
Add tamarind paste or chopped tomatoes and 2 - 3 tsp thick gashi paste, and gently stir well for 2 - 3 minutes.
Add 100 ml water and salt to taste. Mix all the ingredients well.
Cover with a tight lid and cook on a medium to low heat for 10 - 12 minutes or until the gravy thickens.
At this stage, add marinated prawns and cook again for another 5 minutes until prawns are cooked.