• 6 very fresh mackerel
• 2 pints (1.1 L/5 cups)

First make the Bretonne Sauce. Whisk the egg yolks in a bowl with the mustard and finely chopped herbs. Bring the butter to the boil and pour it in a steady stream onto the egg yolks, whisking continuously until the sauce thickens to a light coating consistency like a Hollandaise. Keep warm in a flask, or place in a pottery or plastic bowl (not stainless steel), in a saucepan of hot but