Bring the tomatoes, ground cumin, turmeric, salt and water to a boil in a saucepan; reduce heat to medium and maintain a simmer.
Heat the oil in a frypan over medium-high heat; cook the fish in the oiled frypan until golden brown, 2 to 3 minutes per side. Transfer the fish to the saucepan.
Heat a separate frypan over medium heat and toast the mustard seeds, cumin seeds, fennel seeds and fenugreek seeds until fragrant, just a few seconds. Stir the spices into the saucepan. Simmer until the flavours integrate, about 10 minutes more. Serve hot.