Clean Gambooge in clear running water and soak it in little warm water minimum for half an hour. Keep it aside. Cut all the fins and scales of sardines with a knife, rub it with little salt in an earthen ware and clean them in clear running water. Keep it aside. Take a small bowl and all the spice powders and pour little coconut oil or little warm water to form a paste. Grind it in a mixer and keep it aside.
Heat coconut oil in an clay cooking pot / manchatti. Add mustard seeds. When it starts crackling, add the fenugreek seeds. Then add the dry red chilli and sauté it. When these seeds get slightly brown in clour, add the finely chopped shallots and one spring of curry leaves. Shallots have to get the golden colour. Then add chopped ginger and garlic followed by green chillies. Sauté it for few seconds till green chillies changes its colour and garlic gets light yellow colour. Add in the tomato piece and sauté it for few seconds.
Now add the ground spice powders paste you made in advance. Sauté it well till you get a nice aroma and the oil starts separating from the pan. Add the water, salt and the other curry leaves. Bring it to a boil.
When it started boiling add the cleaned sardine pieces. Cover the vessel with its lid and simmer the stove. Let it cook for 10 minutes or until the fish is cooked and oil starts floating. If you want to reduce the gravy, cook for few more minutes keeping its lid removed.
Your Sardine Fish Curry / Keralas Mathi Curryis ready to serve.
Serve it with hot rice or kappa vevichathu. It goes well with Kappa Vevichathu and Appam or Kallappam.