Sea bass en croûte with mousseline sauce

Method

1. For the sea bass: place a 60 x 40cm baking tray in the fridge to chill. If you haven’t persuaded the fishmonger to do so for you, prepare the sea bass. Scrape off the scales with the back of a knife and cut off the side and dorsal (back) fins with scissors. Lay the fish on one side and, using a filleting knife, open it up through its back to the belly gliding the knife over the bones.

2. Repeat on the underside of the bones. Use the tip of the scissors to cut through the backbone close to the head and the tail. Pull out the whole backbone; the guts will come away with it. Place the tip of the scissors inside the fish and remove the gills, then trim the end of the tail.

3. Rinse the sea bass under a trickle of cold running water. Sponge it inside and out with kitchen paper, place on a tray, then season lightly with salt and put the fennel fronds (or thyme) into the cavity. Cover the fish with cling film and refrigerate.

4. Preheat the oven to 200C/gas 6. Lightly flour the work surface and roll out 40 per cent of the pastry into a rectangle about 50 x 15cm. Roll the pastry loosely round the rolling pin and unroll it onto the chilled 60 x 40cm baking tray.

5. If using, make the crêpes by mixing the crêpes ingredients in this manner: put the flour into a bowl and make a well. Pour into the well one-third of the milk, the eggs and salt and freshly ground black pepper, to taste. Mix lightly with a whisk to make a smooth batter; then pour in the rest of the milk and mix thoroughly. Pass the batter through a fine metal sieve into a clean bowl. Cover the bowl with cling film and set aside to rest for at least 30 minutes. When you are ready to cook the crêpes, stir the chopped herbs into the batter and lightly grease a 26-30cm frying pan with a little of the clarified butter. Give the batter another stir, then ladle in just enough to cover the base of the pan. Cook quickly for about 1 minute, then turn the crêpe over with a palette knife and cook for barely a minute. Place the cooked crêpe on a plate with a layer of greaseproof paper on top. Keep stacking them on the plate, layering a piece of greaseproof paper between each one to prevent them sticking together.

6. Lay the two crêpes along the centre of the pastry, overlapping them slightly so that they cover almost the whole length. Cover with 4 blanched spinach leaves, lay the sea bass on top and bring the sides of the spinach up over the fish. Cover with the rest of the leaves to wrap the fish completely in spinach. Now bring the crêpes up over the fish and fill any gaps with pieces cut from the remaining crêpes. Don’t overlap them more than a mere fraction; the fish should be enclosed in a single layer of crêpe. Brush the puff pastry around the fish with eggwash (see Cook's note).

7. Roll out the remaining pastry into a 55 x 20cm rectangle, roll it loosely round the rolling pin and, starting at the head, unroll it over the fish towards the tail to cover it completely. Leave to rest in the fridge for 20 minutes.

8. With a very sharp, small pointed knife, cut off the excess pastry around the fish, leaving a 2-3cm wide border. Press lightly all round the border with your fingertips to seal it. Brush the whole surface of the pastry with eggwash. With the knife tip, make very light incisions on the pastry to look like fish scales. Draw eyes on the head and mark rays on the heat, tail and all round the border. Bake for 10 minutes, then lower the oven setting to 180C/gas 4 and cook for another 25 minutes until golden-brown and cooked all the way through.

9. For the mousseline sauce: in a saucepan over a low heat, mix the wine vinegar, 4 tbsp cold water and the crushed pepper. Allow to bubble until the liquid has reduced by one-third. Leave to cool completely.

10. Add the egg yolks to the cold reduction and whisk to combine. Set the pan (on a heat diffuser if you have one) over a very low heat and continue whisking. Gradually increase the heat so that the sauce gradually emulsifies, becoming very smooth and creamy after 8-10 minutes. Don’t let the temperature rise above 65C/150F.

11. Take the pan off the heat and, while still whisking, pour in the tepid clarified butter in a steady stream. Season with salt, to taste. Whisk the cream in a mixing bowl until soft peaks form when the whisk is removed from the bowl. Fold the cream into the sauce and then at the last moment, stir in the lemon juice.

12. To complete the sea bass: use a wide palette knife to slide the parcel carefully onto a wire rack. Leave to stand for 5-10 minutes before serving. Brush the pastry with a little clarified butter to give it an extra shine.

13. Present the fish whole on a platter to impress your guests, then cut into slices using a very sharp serrated knife. Lift onto individual plates and pour on some mousseline sauce. Serve the remainder of the sauce in a sauceboat.