Marinate the fish in turmeric powder, salt and lemon juice for 20 minutes. Heat the oil in a kadahi and deep fry both the sides of the fish pieces until slightly brown, drain and keep aside.
Grind the mustard seeds, jeera seeds and garlic cloves to make a fine paste. Now once the paste is ready, add in it turmeric powder and red chillie powder . Now heat up the remaining oil, and put the methi seeds to crackle and mix the paste in the oil and stir fry for a few minutes. Add the finely chopped tomatoes and cover it and cook in a medium flame.
Add the water and salt for gravy and boil it. Now add the fried fish pieces in the gravy and cook for 5 minutes. Garnish with coriander leaves and serve hot fish curry with rice.