Place the young turbot, which has been seasoned inside and out, on a buttered baking dish with a chopped, medium-size onion and a bouquet garni. Sprinkle with several spoonfuls of white wine or of fish stock plus white wine. Cook in a slow oven.
Drain the fish. Transfer it to a serving dish.
Spoon over the fish a white wine sauce prepared as follows:
Cook down by two thirds, a deciliter and a half of fish stock made with white wine. Let it cool to lukewarm and add two egg yolks beaten over low heat with a wire whisk until they become frothy and lemon-colored. When they have reached the proper consistency, gradually beat in 150 grams of melted butter. Beat briskly as the butter is added. Season the sauce. Add a few drops of lemon juice. Put it through a fine sieve. Spoon this over the fish. Place briefly in a very hot oven until nicely glazed.