Poached Whole Fish with Onion Sauce
For the fish: Rinse the fish. Bring a large pot of salted water to a boil. Season generously with vinegar. Add the fish and simmer over medium heat for 15-20 minutes.
Meanwhile, for the sauce: Peel the onions and cut into rings. Melt the butter in a large saucepan and sweat the onions until softened. Sprinkle with flour and cook briefly, stirring. Pour in the white wine and fish stock and simmer for 3-4 minutes. Season with salt and pepper. Rinse the parsley, shake dry and pluck the leaves. Stir into the sauce.
Carefully remove the fish from the poaching liquid and drain briefly. Serve with the onion sauce.