In a bowl, combine turmeric powder, salt, black pepper, and lemon juice.
Coat the fish fillets with the marinade and let it sit for at least 30 minutes.
Heat the vegetable oil in a large pan over medium heat.
Add mustard seeds, cumin seeds, and coriander seeds. Cook until they start to pop.
Add the chopped onion, garlic, ginger, and green chilies. Sauté until the onion is translucent.
Stir in turmeric powder, paprika, and sugar. Cook for a minute.
Add chopped tomatoes and cook until they break down and form a sauce.
Pour in the white vinegar and water. Bring to a simmer.
Gently add the marinated fish fillets to the simmering sauce.
Cover and cook for about 10-15 minutes, or until the fish is cooked through and tender.
Check for seasoning and adjust salt or sugar if needed.
Stir in chopped cilantro leaves.
Enjoy your meal!