Thinly slice onion. Heat 1 tablespoon oil in a pan. Add sliced onion, curry leaves and some salt. Salt absorbs moisture from onion and helps onion to get cooked fast.
Saute onion until it turns brown. Add oil if needed. ** Onion should be well sauteed in this recipe. So never compromise on this step :)
Grind ginger and garlic into fine paste. Add ginger and garlic paste. Add 1 teaspoon oil and saute it well.
Add turmeric powder, chili powder, pepper powder, coriander powder, and garam masala to the above mix. Add 2 teaspoon oil and saute it well until it releases a pleasant aroma.
Slice tomatoes. Add it to the above gravy and mix well. Cover and cook for around 5 minutes or until tomatoes are cooked. Add enough salt. Add cocum / kudampuli.
Add cleaned prawns and mix well. Cover and cook until prawns is done.
Prawns itself has water in it. Ideally, prawns should get cooked by the time water gets dried up. I like to add some coriander leaves at this stage. It's optional. Remove cocum. Otherwise, it might become over sour.
Once prawns is cooked, open the lid. Saute it until the consistency you desire. Serve it with rice, pulao, ghee rice, Chapati/any Indian bread. Enjoy :)