Cut onion lenghtwise.(its not necessary to cut it thin), then take a pan put oil and saute onion into this oil until it is soft and transparent( dont need to brown the onion).
Then remove it from oil and put it in blender and add coriander leaves and green chillies and make a fine paste out of it. Then in the previous oil put this blended masala. Add garlic paste,zeera pdr, saffron(soaked and crushed in warm water) black pepper pdr,salt.
Saute the masala until oil comes out then add water according to u(generally it has a thick gravy). When it boils then add deshelled and deveined prawns to it and cook until praws are done( dont cook it for longer time otherwise it will become hard). It goes well with rice. In the same preparation if u want then at last u can add some cream and it goes well with chapatis or naan.