Take the Chatti (earthenware) or a pan, in which you have planned to cook the fish. Arrange the fish on the chatti /pan such the bottom of the pan is covered with fish. Now sprinkle the curry leaves, salt and the soaked Kodumpuli / Gambooge on top of the fish.
Make the gravy|:| Heat oil in a kadai or pan add splutter mustard & fenugreek seeds.
Now add the thinly sliced shallots, chopped ginger & Garlic are saute for 3 minutes.
Now reduce the heat and add chilli powder and turmeric powder, fry for a minute and then immediately add half of the water (water used to soak Kodumpuli / Gambooge).
Allow it to fry for 2 minutes now add the remaining water and allow it to boil. Cook the fish. Put the chatti/pan on heat with fish and pour the gravy on it. Combine well.
Make sure that fish pieces do not get broken in the process; it will be safer to rotate the pan to ensure that fish & gravy are mixed well.
Add water if required, such that the fish is covered with the gravy.
Cook it in low heat with lid on till the fish is done & gravy becomes thick. (Rotate the chatti/pan in between to help the fish and gravy get mixed well) This will take around 20 -25 minutes on low flame. Serve with rice (Kerala red rice).