Clean & wash the sardines. Make random slits of the fish. In a small bowl, make a smooth paste of the red chilli powder, half of the turmeric powder, pepper powder, vinegar, and salt. Marinate the fish with the prepared paste and leave it aside for 30 mins. Heat oil in a large pan. Fry the fish until dark brown on both sides. Drain them onto a plate and keep it aside.

In the same pan, add oil. Splutter mustard seeds and curry leaves. Add the sliced onions, green chilies, ginger, and garlic. Saute until the onions are translucent and soft. Add the remaining spice powder and fry until the raw smell goes away for 2-3 mins. Add the coconut milk and simmer for 2-3 mins. Add the fish fish and the tomato slices.

Drizzle some vinegar and add salt to taste. Garnish with curry leaves. Switch off the flame before the gravy starts to boil. Serve hot with appam or steamed rice.