If you are using Mustard Seeds powder like me, then soak it with some salt in warm to tepid water for 10 minutes.
Take Poppy seeds and buzz them with some water to make a smooth paste of it. Now, add the soaked mustard powder in it and pulse it together with some turmeric powder, lemon juice and a green chili.
Take a glass baking dish and put it in your steamer. Now, dip each piece of fish in the mustard-poppy paste and lay it in the baking dish. Once all the fish pieces are laid side-by-side, pour the remaining paste over the fishes.
Drizzle some mustard oil on top and lay 2 green chilies slit in halves.
Now, steam it for 7-10 minutes or till the fish has been cooked properly. Once done you will see that the flesh has turned white inside.