Take the chopped onions in a mixing bowl, add 3 tbsp of mustard oil, green chilies and a little salt. Give it all a hearty mix for 7-8 minutes or so. The onions will slowly become little pale in colour. Keep it covered for 10 more minutes.
In a flat pan, make a bed of the above onion-chilies-mustard oil mix. Switch on the flame and cook for 2 or 3 odd minutes.
Gently place the Ilish steaks on the onion bed. Cook over low flame for 3-4 minutes.
Add ½ a cup of warm water, cover the pan, cook over a medium flame for 7-8 minutes.
Adjust seasonings, finish it off with a generous drizzle of mustard oil. Serve hot with steamed rice.