Melt the butter in a pan, add the onions and fry until golden.
Drain on kitchen paper.
Season the flour with salt and pepper and use to coat the herrings.
Fry in the fat remaining in the pan until well browned.
Arrange on a warmed serving dish.
Return the onions to the pan and pour in the wine.
Bring to the boil and cook for 2 minutes.
Sprinkle in the parsley, then pour over the herrings.