Place fish slices in a bowl and marinate with turmeric powder, ¾ tsp red chilli powder, salt, garam masala powder and tamarind paste for about 15 minutes.
Heat sufficient oil in a non stick pan.
Apply rice flour to the marinated fish slices, place them on the pan, cover and shallow fry, turning sides till they turn golden on both sides.
Place the onion slices on a serving plate, sprinkle some of the remaining red chilli powder and a little chaat masala over them. Arrange lemon slices over them.
Drain the fish slices on absorbent paper. Transfer the slices onto the plate. Sprinkle some chaat masala and red chilli powder over them and serve hot.