Mix all the ingredients except the oil and marinate for about 1/2 an hour.
Heat the oil in a kadahi/wok, and when hot ( a piece of fish put in should come up at once), add as many pieces as come in comfortably. Fry over high heat, till lightly coloured.
Drain the fish from the fat, and keep aside over a colander (to drain excess oil). Let the oil get hot again, before you do the same with the rest of the pieces.
Before serving, heat the oil, and again over high heat, add the pieces of fish to it, press them down a little and let fry till cooked through and crisp on the top. (When cooked through they are opaque when you cut through.
Drain on absorbent paper and serve hot sprinkled with chaat masala and with lemon wedges on the side.