In a bowl, take 1 kg of Rohu fish, cut into relatively thicker pieces. Wash them nicely and keep aside.
Now, in a mixer grinder, take 10 pieces of peeled garlic cloves, 1 piece of ginger (1-inch size, cut into small pieces), 5 pieces of green chilies (whole, without stalk), and squeeze out 4 tsp of lemon juice. Make a fine paste without adding any water to it.
Put this masala paste to the washed fish pieces in the bowl along with 1 tsp of salt and mix them nicely with your hand. Cover the bowl nicely and put it in the fridge for about half an hour to marinate.
Now, in a plate, take 5 tbsp of gram flour, 3 tbsp of rice flour, 2 tbsp of Kashmiri red chili powder, 1 tsp of cumin seeds powder, ½ tsp of garam masala powder, 1 tsp of chaat masala powder, ½ tsp of carom seeds (ajwain), ½ tsp of salt and mix them all nicely.
Put each fish piece on this mixture and coat them nicely on both sides. Repeat the process to coat all the fish pieces, keep them on a plate and keep aside for 2 to 3 minutes.
In the meantime, heat an adequate amount of cooking oil on a medium flame. When it is hot, put in each of the coated fish pieces carefully. Let them cook on one side for about 4 to 5 minutes.
Flip them carefully with a pair of tongs, one by one, after each become nicely crispy and golden on one side. Let it cook on the other side for another 4 to 5 minutes.
Remove them on a plate when done. Garnish with a few chopped coriander leaves and squeeze a bit of lemon juice on them.
Your crispy and spicy homemade masala fish fry is now ready to enjoy. Serve them hot with onion rings and lemon wedges.