Cut the cleaned fish in half lengthwise, debone it, then tie it back into shape carefully with kitchen string.
Salt it well, and let it sit for about 20-30 minutes.
Meanwhile, prepare the poaching liquid: bring the water to a boil with the vegetables, onion, peppercorns, bay leaves, and vinegar, and cook for 10 minutes, until nicely tangy.
Let the liquid cool slightly, then add the fish.
Poach gently, at a bare simmer, for about 30 minutes.
Let the fish cool completely in the poaching liquid.
Carefully remove the fish, and chill it fully on ice or in the fridge.
Slice and arrange on a platter, garnishing to taste.
Serve with mustard sauce or mayonnaise.
Based on: Malatinszky Fanny szakácskönyve (a classic Hungarian cookbook), modernised. Calories and protein are estimated.