Sprinkle the cod with the asafoetida, salt and sugar, then set it aside in the fridge for an hour to firm up.
Add the gram flour, yoghurt, milk, chilli powder, onion salt, garlic powder, amchoor, cumin, turmeric and ajwain seeds to a bowl and whisk to form a thick batter.
Remove the fish from the fridge and cut it into bite-sized chunks. If your cod loin has a "seam". Remove that first by cutting along the seam and then cutting it into strips. Finally, slice across the large loin piece to form nice medallions.
Fill the base of a wok with 3-4cm (1½-1¾") of cooking oil and heat it to a temperature of 170°C or 340°F. I use a sugar thermometer to measure the temperature.
Place the cod in the batter and carefully coat. This is a thick batter, so your hands will get mucky!
Drop the fish into the oil individually and fry for 4-5 minutes before placing it on kitchen paper to soak up any excess oil. This is best done in two batches with this size of fish. You can keep the fish warm in a moderate oven whilst you finish the second batch.
Sprinkle with chaat masala to finish whilst the fish is still hot.