In Bengal, you would use a local carp called rohu in this recipe, but using a filleted meaty white fish in its place such as cod or halibut works just as well.
2 Large Fresh Tomatoes, chopped
1 tablespoon Ground Turmeric
2 teaspoons Ground Cumin
1/2 teaspoon Kosher Flake Sea Salt
2 cups vegetable or fish stock
1 tablespoon canola or vegetable oil
1/2 teaspoon Brown Mustard Seeds
1/2 teaspoon Nigella/Black Caraway Seeds
1/2 teaspoon Cumin Seeds
1/2 teaspoon Fenugreek Leaves
2 Pounds Cod Fillets, cut into thick strips
Place tomatoes, turmeric, ground cumin, salt and stock in large stock pot and bring to a boil over medium heat.
Meanwhile, heat vegetable oil over medium heat. When warm, add mustard, nigella and cumin seeds and toast until fragrant, about 30 seconds. Add fenugreek and continue cooking, stirring constantly, for 30 seconds more. Add spices to stockpot, stir and simmer 10 minutes to allow flavors to meld.
Add cod to pot and simmer in sauce until cooked through and no longer translucent, about 5 minutes. Serve hot.