Add the marinade ingredients to the prawns, mix thoroughly, and refrigerate for 2-4 hours.
Add oil to a heavy-bottomed pot and throw in the methi seeds. Select a pan that will accommodate all prawns pieces in a single layer.
Place pot on medium heat, and raise temperature slowly until the methi seeds start popping and becoming very fragrant. Do not allow these seeds to burn or else you will have to start all over again! Methi is very nice ONLY if it does not scorch.
Reduce heat to low and add chopped onion, cover, and fry until onion becomes soft. Add garlic, chilies, turmeric powder, Dhana Jeeru powder, curryleaves and mix in. Cover again and fry mixture gently for about a minute, ensuring it does not burn.
Add chopped tomatoes and salt, and increase heat to medium. Cook covered until tomatoes have softened.
Spread pan contents evenly on bottom, and then place prawns to sit on this as a single layer. Cover and let prawns cook until it starts to firm up enough so that the pot contents can be stirred VERY GENTLY. Too much stirring will cause the prawns to break apart.
Cook until liquid releases. Now remove cover, sprinkle the Spring Onions on top and let liquid evaporate, stirring a few more times until a fair amount of the thick sticky gravy has formed, but not completely dried out. All ingredients should be cooked by now.
Alternatively, if you want a soupy dish, add the desired amount of boiling water, and let cook until water is well incorporated into the gravy.
OK, you can now go ahead and taste. Adjust salt if needed. Remove from heat.
Sprinkle Green Dhana and Garam Masala. Cover and let sit for about an hour for flavours to meld.
Just before serving, reheat gently without stirring.