Heat oil in a pan and add the green chilli and onion. Fry till the onion turns lightly brown.
Add in the ginger garlic paste and fry till the raw smell disappears from it.
Add in the turmeric powder, red chilli powder, coriander powder and stir.
Throw in the tomatoes and cook till the tomato turns soft and pulpy.
Add in the prawns and cook on low flame till the prawns are cooked and all the masala is coated to the prawns well. (There won't be any necessity to add water as the prawns leave water while getting cooked. But if you feel that they are turning dry, add in some water.)
Finally add salt and coriander leaves. Mix well and switch off the flame.
Serve hot with plain rice and dal or as a side dish to roti.