Wash the mackerels thoroughly. Apply some salt, turmeric powder and lemon juice. Keep aside for 10 minutes.
Marinate the mackerels with the rest of the ingredients except rice powder preferably in a refrigerator for about an hour.
Heat the oil till it is very hot. Roll the mackerels in rice powder one by one and and deep-fry in batches on medium heat till they are crisp.
Garnish with onion rings and lemon wedges.