For the potatoes, boil the potatoes in a pan of salted water for 12-15 minutes, or until tender. Drain well, then return to the pan and add the butter. Stir until the butter has melted and coats the potatoes. Season, to taste, with salt and freshly ground black pepper. Keep warm.
Meanwhile, for the Sandefjord sauce, bring the cream and star anise to the boil in a saucepan. Reduce the heat until the mixture is simmering, then continue to simmer until the volume of liquid has reduced by half.
Gradually stir in the pieces of butter, one at a time, stirring continuously, until all of the butter has been fully incorporated into the cream mixture and the sauce has thickened. Remove the pan from the heat, then stir in the chives, dill and lemon zest. Season, to taste, with salt and freshly ground black pepper. Keep warm.
Meanwhile, for the baby leeks, bring a separate pan of water to the boil, add the leeks and cook for 3-4 minutes, or until tender. Drain well, then return to the pan and add the butter. Stir until the butter has melted and coats the leeks. Season, to taste, with salt and freshly ground black pepper. Keep warm.
Meanwhile, for the turbot and langoustines, heat 60g/2½oz of the butter in a frying pan over a medium heat. When the butter is foaming, add the turbot fillets, in batches if necessary, and fry for 2-3 minutes on each side, or until golden-brown on both sides and just cooked through.
Bring a large pan of salted water to the boil, add the langoustines and boil for 3-4 minutes, or until pink and cooked through. Drain well. Set four whole langoustines in their shells aside as a garnish. Remove the shells from the remaining langoustines using scissors or a sharp knife, but leave the heads and tails intact (discard the shells).
To serve, place one turbot fillet into the centre of each of four serving plates. Place three of the peeled langoustines over the turbot fillets. Spoon the buttered potatoes and baby leeks alongside. Drizzle over the Sandefjord sauce. Garnish the edge of each plate with one whole langoustine.
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