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1
Submerge the pike pieces in 500 ml of cold milk and dégorge in the refrigerator for 2 hours. This step purifies and whitens the flesh. Drain and pat dry — the flesh should be visibly pale and firm, with no grey tinge remaining.
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2
Make the panade: melt 60 g butter in a saucepan over medium heat, add the 250 ml milk, and bring to a simmer. Add 100 g flour in one addition and beat vigorously with a wooden spoon until the paste pulls cleanly from the sides of the pan. Remove from heat, beat in the 2 eggs one at a time, and continue beating until the paste is absolutely smooth and glossy. A single lump here will surface in the finished quenelle. Allow to cool completely.
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3
Process the dried pike flesh in a food processor for a full 5 minutes until reduced to a very fine paste. Add the cold butter pieces, the 2 eggs, and the cooled panade; process for a further 2 minutes. Season with salt. The farce should be homogeneous, satiny, and hold a shape when pressed between two fingers.
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4
Whip the 200 ml cold double cream to soft peaks. Fold the cream into the farce by hand, in three additions, with a flexible spatula. Test a single quenelle before shaping the batch: lower one shaped oval into barely simmering salted water and poach for 12–15 minutes. It must puff to nearly double its original size and hold together cleanly — this is where the dish lives or dies. If it collapses, work in one more egg; if it is dense, fold in a little more cream.
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5
Pound the crayfish shells thoroughly in a mortar or heavy-duty food processor. Combine with 200 g butter in a small saucepan over gentle heat and warm slowly, stirring, until the butter takes on a vivid coral-orange colour and the kitchen smells of concentrated shellfish. Do not allow the mixture to boil. Strain through fine muslin, pressing hard to extract every drop of coloured fat; discard the shells. Reserve the crayfish tails separately.
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6
Make the béchamel: melt 40 g butter, add 40 g flour, and cook the roux for 1–2 minutes without colouring. Whisk in the warm 500 ml milk progressively until the sauce is smooth and thick. Stir in the 150 ml cream and the splash of cognac. Beat in the crayfish butter, spoonful by spoonful, over low heat — the sauce should turn a uniform coral-orange and coat the back of a spoon evenly. Add the reserved crayfish tails. Keep the sauce hot.
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7
Shape the remaining farce into elongated ovals using two large wet spoons. Poach in gently simmering — never boiling — salted water for 12–15 minutes. The quenelles are done when they puff to nearly double and feel set yet trembling when nudged with a spoon: a barely perceptible quiver at the surface is the correct signal.
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8
Lift the quenelles with a slotted spoon and drain on a cloth. Arrange two or three per warmed deep plate, ladle the hot Nantua sauce generously around and over them, and garnish with the reserved crayfish tails. Glaze briefly under the salamander if desired, until the sauce just begins to blister at the edges. Serve immediately — the sauce must be hot and flowing, coating each quenelle in a continuous coral pool.