Heat 2 tbsp oil in a vessel and add 1 medium chopped onion. Fry the onion for a minute or until translucent.
When the onion is translucent, then add the masala paste and fry for at least 2 minutes to get rid of the raw smell of the ingredients.
Now rinse the blender jar with water and add this water to make the curry. Also, add the tamarind juice and then cover the vessel, and bring the curry to a boil.
Next, add the tuna fish to the curry and allow it to cook for another 5 minutes or until done. Check for salt and add if needed. I also like to add a few green chillies to the curry at this stage.
Switch off the flame once the fish is cooked. Serve Tuna Fish Curry with steamed rice or rice rotis.