Heat oil and cook tomatoes, green chilies in one tablespoon oil, until tomatoes becomes soft.
Fry prawns in a separate pan with 2 tablespoons oil for 7 to 8 minutes on medium flame, water will become dry.
Roughly crush cumin, coriander, green chilies and black pepper in grinder and make granulated powder (dont make fine powder).
Add prawn in cooked potatoes with salt and cook them for three minutes on medium heat.
Sprinkle grinded powder on top of prawn karai, julian ginger and green chilies, cover it and simmer for three minutes.
Serve with roti, nan, or rice.