Clean the fish properly with running water. Add 2 tsp lemon juice and let it stay for 5-7 minutes. Wash the fish and drain out all the water. Mix 1 tsp salt, 1 tsp garlic paste, 2 tsp cornflour and 1 tsp turmeric powder. Rub this paste on the fish and keep aside. Heat some oil in fry pan on high flame and add a tsp of mustard seeds when the oil is hot. Allow the seeds to sputter for few seconds. Reduce the heat to a low flame. Add the fish pieces and increase the flame. Fry on both sides till dark golden brown, about 2-4 minutes on each side. Remove from the flame and fry remaining pieces in the same way. Garnish with chat masala, onion rings and lemon. Serve hot with rice or chapati and dal, and enjoy!