Wash and clean the rohu fish pieces and then marinate the fish with 1/2 teaspoon of turmeric powder, salt to taste, and 1/2 teaspoon red chilli powder or taste.
Keep marinated fish aside for 10-15 minutes.
Meanwhile, take a grinding jar and add mustard seeds, cumin seeds, dry red chillies, and garlic cloves.
Add water as required and make a smooth paste out of it and keep it aside for later use.
After the fish marinated for 10-15 minutes, take a frying pan and turn on the flame.
When the pan is hot, add 3-4 tablespoons of mustard oil or any kind of cooking oil.
When oil is smoking hot, add the marinated fish pieces and shallow fry them on a medium flame until they are light golden brown in colour on both sides. Do not overcrowd the pan. (If you want you can deep fry the fish).
Transfer the shallow fried fish pieces to a plate and keep them aside for later use.
Take a kadai and turn on the flame.
When kadai is hot, add 2-3 tablespoons of mustard oil or any kind of cooking oil.
When oil is hot, add panch phutan and let it crackle.
Add chopped onions and saute the onions on medium to high flame until the onion turns golden brown in colour.
Add chopped tomatoes, salt to taste, and 1/2 teaspoon turmeric powder and mix well.
Cover the lid and cook the tomatoes on medium to high flame until tomatoes are soft, mushy, and well-cooked by stirring occasionally.
Add mustard paste, which we made previously out of mustard seeds, cumin seeds, garlic, and dry red chillies.
Mix all the ingredients well and cook the masala on a low to medium flame for 4-5 minutes, do not overcook the mustard paste as it will taste bitter.
Add water as required, dry mango (ambula), and slit green chillies and mix well.
Check for the seasoning and adjust accordingly.
Increase the flame and let the water come to a boil.
When water comes to a boil, lower the flame and simmer the gravy for 5-7 minutes by stirring often.
Add shallow fried fish pieces and mix very gently so that the fish pieces do not break.
Cover the lid and simmer the machha besara for another 4-5 minutes.
Add chopped coriander leaves and mix very gently.
Turn off the flame and transfer this machha besara to a serving bowl and serve hot with hot steaming rice.