Recipe originally published here:
Take one pound fish, cut into large chunks, and rub with turmeric and salt. Fry in a hot pan with two or three green cardamom (whole) tossed in, till the flesh has opaqued. Set aside.
Sauté one small onion (or two or three shallots), finely chopped, in the same pan, with a pinch of cumin added. When the onion is nicely golden and the cumin darkly fragrant, splash with water and cook down till the oil separates.
Add one cup of freshest possible yoghurt into which you have beaten one teaspoon black mustard seeds (sarson ka beej) which have been toasted and coarsely ground, one teaspoon cayenne (lal mirch), a pinch of turmeric (haldi), and two tablespoons of besan flour. The besan flour ('gram flour') is EXTREMELY important! If you do not add it to the yoghurt, the yoghurt will curdle in the pan.
[NOTE: this blogger will also whip a goodly pinch of corn or tapioca starch into the yoghurt, as that also helps prevent watery separation, and further allows greater use of oil.]
One or two chopped Roma tomatoes may be added, as well as split green chilies.
It is up to you, and your personal sense of yumminess.
Anyhow, once the yoghurt mixture has reached a boil, stir it, slide the fish chunks into the sauce, and turn the heat down to simmer.
Add a pinch of freshly ground white pepper and an even smaller pinch of cinnamon (in lieu of Sindhi Garam Masala, which you do not have), cook for a few minutes more, then garnish liberally with minced cilantro (coriander leaf), and serve with a heap of white rice alongside.
Ideally, you would use mustard oil (sarson ka tel) in cooking this dish, but it is quite possible that you do not have that, which is a pity. But do not worry, other cooking greases also can.