Onions, leeks, celery and fennel are washed and cut. Place in a deep pan and pour 3 liters of cold water. Put on the heat and boil the broth should vigorously for 10 minutes, then add the wine, vinegar, salt and pepper to taste. The fire immediately reduced and put the fish. Fish boil 12 minutes without boiling. It must acquire specific blue and take the flavor of the vegetables. Drain and place in a heated vessel. Melts gently 100 g butter - to become creamy and pour over fish. Serve garnished with boiled potatoes sprinkled with chopped parsley and decorated with lemon slices.