Soak the bottlegourd leaves in boiling salted water for 10-15 odd minutes. Shake off excess water, keep aside.
Soak the yellow mustard seeds and black mustard seeds in warm water for 15 odd minutes.
Drain the water. Make a smooth paste with 3-4 green chillies, a pinch of turmeric powder and a couple of grains of rice, keep aside.
Add the mustard paste, turmeric powder, mango pickle oil, mustard oil and the remaining green chilies to a bowl. Give it a hearty mix.
Gently place the fish steaks in the bowl, sprinkle in the salt, give it another loving mix. Keep aside for 10-15 minutes.
Arrange the bottlegourd leaves as shown in the picture, place one hilsa steak on the leaves, top with a little marinade.
Wrap from all four sides to make a parcel, secure with twine. Repeat for each of the remaining fish steaks.
Steam the bottlegourd wrapped ilish in a steamer for 15 odd minutes.