Heat 5 tablespoons of mustard oil in a heavy pan until it changes color.
Add ¼ teaspoon each of salt and turmeric powder to the oil and stir.
Fry the fish pieces one at a time, making sure the skin gets nice and crispy. Each piece should take about a minute. You can fry two or three pieces together if you prefer.
Finally, in the remaining oil, add three dried red chilies, along with 1 teaspoon of Panch Phoron (Bengali Five Spices). Cook this for about a minute.
First, add the potato wedges and fry for about a minute.
Next, toss in the chopped onion and lightly fry over medium heat for roughly three minutes.
Now, add the chopped tomatoes, stir everything well, and cook for another minute.
Throw in the green chili slices, using more or less depending on your spice preference, and cook for one minute.
Stir in the chopped coriander leaves.
Add ¼ teaspoon each of salt, turmeric powder, and red chili powder, then mix well. Reduce the heat to low and cook for three more minutes.
Now, pour in two cups of water, using the bowl that held the fish to capture any leftover spices.
Taste and add more salt if needed; I added about ½ teaspoon salt.
Cover the pan and let it cook on low heat for five minutes, or until the potatoes are soft and cooked through.
Gently add the fried fish and stir carefully.
Finally, cook over high heat for three minutes, allowing the curry to bubble and the oil to rise to the top.