Soak the yellow and black mustard seeds in warm water, some 15 minutes or so. Drain from water, add a pinch of rice, 3-4 green chillies and the garlic cloves, blend to a smooth paste.
Move the mustard paste to a tiffin box, add the onion paste, remaining green chillies, turmeric powder, salt and mustard oil, mix well.
Gently place the fish in the tiffin box, coat well with the paste. Close the lid of the tiffin box.
Steam for 12-15 minutes. Remove from flame, allow to to rest for 10 minutes or so.
Carefully open the lid of the tiffin box, finish with a drizzle of some more mustard oil if you like the zing. Serve hot with rice.