Cut the fish into small pieces and wash thoroughly. Add turmeric powder and salt to it for marinating. Make a paste of mustards, tomatoes and garlic. Peel the potato and cut in to medium squares. Pre-heat a pan on medium flame. Put oil and fry the pieces of fish for 4 to 5 minutes. Keep the fried fish aside. Now add some more oil to the pan. Add cumin seeds  and mustard ½ tea spoonful each. When the cumin seeds and mustard start cracking add the sliced potatoes. Add tomato/mustard/garlic paste, turmeric powder, red chili powder and salt to it. Also add 3 cups of water. Keep the pan on medium flame for about 10  to12 minutes. When the curry starts boiling add the fried fishes and boil it for another 3 minutes. Maccha Besar is now ready to eat with rice. Garnish it with coriander leaves.