1 large brill or Black Sole weighing approx, 1.4kg
2 egg yolks
110 g butter
1 teaspoon of green tarragon flavoured mustard or Dijon mustard
½ teaspoon vinegar or lemon juice
1 generous tablespoon of chopped herbs, a mixture of chives, parsley, chervil, tarragon and a small pinch of thyme leaves
Preheat the oven to 180c / 350f / gas 4
Place the fish, dark side up, on a sturdy chopping board. With a sharp knife, remove the head if you wish. Starting at the top of the fish, near where the head was, cut through the skin, into the flesh, just inside the “frill” and continue all the way around the fish. The cuts on either side of the edge of the fish should meet and overlap at the tail, forming an X. This facilitates the easy removal of the skin after cooking. Give the fish a good wash being particularly careful to remove the little blood clot at the top of the backbone.
Put 5mm of water into a shallow baking tray or roasting pan and slide in the fish. Sprinkle a pinch of salt over the skin. Place the fish in the preheated oven and cook for 20-30 minutes or until the fish is cooked. . To test if the fish is cooked, insert a knife into the thickest part of the flesh at the head end. The flesh should be white and firm with no trace of pink, and willing to come away easily from the bone.
Watch the water level in the tray during the cooking, topping up the water a little if the pan looks like it might go dry.
When the fish is cooked, you can leave it on the tray in the oven with temperature lowered to 100c / 200f / gas ¼, for up to 30 minutes.
While the fish is cooking make the Bretonne sauce.
This sauce is known as a warm emulsion sauce. What happens here is that when you whisk the hot melted butter in to the egg yolks an emulsion is formed. The two disparate ingredients blend together to form a sauce with a creamy and smooth consistency and that is the way you want to keep them. The sauce is served warm, not hot. It cannot be reheated over a direct heat so you need to keep it warm. So when the sauce is cooked, transfer it into a pryex jug and sit the jug into a small saucepan of water that you then place on the lowest heat you can manage on your cooker. It will sit quite happily like this for an hour.
Place the egg yolks in a small bowl with the mustard and vinegar and whisk to mix. Melt the butter and bring to a boil. Remove from the heat. Now in a very slow drizzle add the hot melted butter on to the eggs, whisking all of the time. The sauce will gradually begin to thicken. This does not mean you can add the butter more quickly as there is only a certain speed at which the eggs can absorb the butter and emulsify. Continue until all of the melted butter including the creamy and sometimes salty bits at the end have been added. The sauce should be the consistency of pouring cream. If it is a bit too thick, Add a little of the fish cooking water from the baking tray to achieve the correct consistency. Keep warm as directed above. Add the chopped herbs to the sauce just before you are going to serve it.
Serve the fish on the bone or filleted, on hot plates with the warm sauce. Garnish with a relevant herb of your choice and pass lemon wedges.