Fry the onions in about half of the oil, let them take color. Add the ginger, then add the curry and some toast, then add water. Stir in the vinegar and sour cream. Let the sauce simmer a little and season, round off with a pinch of sugar, simmer gently until everything is done.
Clean, sour and salt the fish fillets, then dab and flour them. Fry in the remaining oil until the fish reaches the desired cooking point.
Serve with rice and the prepared onion curry sauce.