Season fish with salt and pepper, dust with flour.
Shallow fry in oil until golden. Set aside.
To cook the sauce: In the same pan, add onions and fry until soft. Add vinegar, sugar, water, turmeric, curry powder, and salt. Simmer for 5–10 minutes.
Layer & Chill: Place fish in a glass or ceramic dish. Pour sauce (with onions) over the fish.
Let cool, then refrigerate overnight.