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1
Melt 40 g of butter in a wide pan over medium heat. Add the sliced mushrooms and lemon juice. Sweat for 10–12 minutes, stirring periodically, until the mushrooms have released all their moisture and the pan is completely dry. The mixture should be dense, dark, and intensely aromatic — no liquid pooling at the base of the pan. Reserve one quarter of the sweated mushrooms for garnish and set the remainder aside.
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2
In a separate, heavy-based pot, melt 40 g of butter over medium heat. Add the flour and stir continuously for 3–4 minutes to form a blonde roux. The roux is ready when it smells biscuity and has turned pale gold; it must not darken further.
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3
Gradually whisk the hot chicken stock into the roux in a steady stream, ensuring no lumps form. Bring to a gentle simmer, whisking throughout, then add the sweated mushrooms. Simmer uncovered for 30 minutes, skimming any impurities that rise to the surface. The soup should reduce slightly and deepen in colour. Purée until smooth, then pass through a fine sieve, pressing firmly to extract all liquid. Return the strained soup to gentle heat.
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4
This is where the dish lives or dies. In a bowl, whisk the 2 egg yolks with 100 ml of double cream. Temper the liaison by whisking one full ladle of hot soup into the yolk mixture, then pour the tempered liaison back into the pot in a slow, steady stream, stirring constantly. Heat gently to 82–84 °C — the soup must never reach a boil after the liaison is added. If it boils, the yolks will scramble and the texture is irreversibly lost. The soup is ready when it coats the back of a spoon in a smooth, even film.
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5
Remove from heat. Stir in 30 g of cold butter, piece by piece, until fully incorporated. Season with salt, white pepper, and a grating of fresh nutmeg. The finished soup should be the colour of pale stone and carry a deep, concentrated fungal perfume. Ladle into warm bowls, garnish with the reserved sautéed mushroom slices, and finish with a drizzle of truffle oil.