DOI ILISH - Doi Hilsha - Hilsha in Yogurt Sauce

It is a traditional Bengali recipe prepared with yogurt and mustard seeds. It is not at all spicy dish but a bit creamy preparation.
Ingredients:
4 pieces of Ilish or Hilsa Fish Steak
1 tbsp Yellow and 1 tbsp Black Mustard Seeds paste
1 tbsp Posto or Poppy Seeds paste
Paste of 3 Green Chillies
1 cup Beaten Yogurt
4 Green Chillies slit
1 tsp Turmeric Powder
3-4 crushed Cardamom
Mustard Oil
Salt and Sugar to taste