Clean and wash mushrooms. Peel shallots and cut them into rings. Heat 1 tablespoon butter in a pan. Sauté the mushrooms well. Add wine and bring to the boil. Season with salt and pepper. Add shallots and laurel.
Pour everything into the fat pan of the oven.
Wash the mackerel, dab dry and season from the inside with salt. Place on the mushrooms, add about 1 l water. Cover the fat pan with aluminium foil. Cook in the hot oven (electric cooker: 200°C/circulating air: 175°C/gas: level 3) for about 30 minutes.
Wash the parsley, put something aside for garnishing. Finely chop the rest. Melt 4 tablespoons of butter at low heat, let it cool down a little. Whip egg yolk, mustard, vinegar, salt and pepper until creamy. Add the butter in a thin stream, beating constantly.
Stir in the parsley. Season to taste the sauce.
Skin mackerels and fillet them carefully (they are very soft and decompose easily). Drain mushrooms and shallots. Arrange the mackerel with mushrooms, shallots and sauce. Garnish with the rest of the parsley.
Baguette tastes good with it. Drink tip: white wine, for example a Muscadet from the Loire.