Rub pomfret with 1 teaspoon turmeric and a pinch of salt.
Mix remaining 1 teaspoon turmeric powder, red chile powder, and 2 teaspoons water in a small bowl to make a paste.
Combine 2 green chile peppers, tomato, mustard seeds, and salt in a food processor; grind into a paste. Thin paste with water if it appears too thick.
Heat oil in a large skillet over medium heat. Add fish; pan-fry until golden brown, 3 to 4 minutes per side. Transfer fish to a plate. Add remaining 2 green chile peppers and onion seeds; cook and stir until chile peppers darken, 1 to 2 minutes.
Pour turmeric paste into the skillet; cook and stir until mixture looks dry, about 1 minute. Pour in chile pepper paste; cook and stir until dry, about 1 minute. Pour in enough water to cover; bring to a simmer. Return fish to the skillet and continue simmering, covered, until fish flakes easily with a fork, about 6 minutes.