Cut the fish into small pieces and wash thoroughly. Add turmeric powder and salt to it for marinating. Make a paste of mustard, tomatoes and garlic. Cut the onion into slices. Pre-heat a pan on medium flame. Put oil and fry the fish in batches of 2 or 3 pieces at a time for 4 to 5 minutes. Keep the fried fish aside. Putting the pan again on medium flame take a little oil . Add cumin seeds and mustard ½ tea spoonful each. When the cumin seeds and mustard start cracking add the sliced onions. Stir a little. Add the tomato/mustard/garlic paste, turmeric powder, red chili powder and salt to it. Also add 3 to 4 cups of water. Keep the pan on flame for about 10 to12 minutes. When the curry starts boiling add the fried fishes and boil it further for about 3 minutes. Maccha Besar is now ready to be served with rice. Garnish it with coriander leaves before serving. (Feedback from a visitor says that she got a strong and pungent taste after preparing the dish. The quantity of mustard should be adjusted in such cases. It is to be prepared in a watery form, in stead of thick gravy one.)