This is such a special centrepiece. I like to serve it with simply cooked vegetables such as steamed baby potatoes, green beans and mangetout.
For the shellfish reduction, melt the butter in a large saucepan and fry the onion, carrot and celery for 3-4 minutes over a medium-high heat.
Add the reserved lobster shells and tiger prawn shells, the chopped North Atlantic prawns and the Cognac and cook for a further 3-4 minutes, or until the shells smell fragrant. Add the remaining shellfish reduction ingredients and bring to the mixture to the boil. Reduce the heat and simmer for 40 minutes.
Strain the mixture through a fine sieve, pressing as many juices from the shells as possible. Return the liquid to a saucepan and boil until the volume of liquid is 100ml/3½fl oz. Pass the reduction through a fine sieve and set aside to cool.
For the mousseline, blend the white fish pieces, salt, shallots, eggs and half of the shellfish reduction in a food processor until smooth. While the motor is running, add the cream in a steady stream until the mixture has a mousse-like texture. Chill the mousseline for at least 1 hour or up to 24 hours.
For the Pithivier, preheat the oven to 220C/425F/Gas 7. Grease a large baking tray.
Roll the pastry out thinly on a lightly floured work surface and cut out a circle of 30cm/12in in diameter. Set aside.
Lightly knead the pastry trimmings, roll them out again thinly and cut out a 28cm/10in circle. Place it onto the baking tray.
Roughly chop the reserved lobster and prawns and stir them into the mousseline.
Spread the mousseline mixture over the smaller pastry circle, leaving a 4cm/2in border around the edge. Brush the edge of the pastry circle with a little water and place the larger pastry circle on top. Seal and crimp the edges. Make a hole in the centre of the Pithivier, then using a small sharp knife, score small curves in concentric circles from the centre of the Pithivier to the edge (without cutting through the pastry). Brush the Pithivier all over with the beaten egg. Bake in the oven for 35-40 minutes, until crisp and golden-brown. Remove from the oven and set aside for 15 minutes before cutting.
Meanwhile, for the sauce, warm the remaining shellfish reduction and the double cream for 5-6 minutes then remove from the heat. Whisk in the butter then stir in the basil. Season, to taste, with lemon juice.
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