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1
Place the dried cèpes in 150 ml of warm water and leave for 30 minutes. Lift the mushrooms out, squeeze gently, and set aside. Pass the soaking liquid through a fine sieve lined with muslin and reserve. The liquid should be dark and fragrant with no trace of grit.
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2
Heat a wide sauté pan over high heat until very hot. Add one third of the clarified butter. Sauté the fresh mushrooms in uncrowded batches — cèpes first, then girolles, then pieds de mouton, then trompettes — adding fresh butter between batches as needed. Each batch is done when the cut faces are deeply caramelised, dry, and pulling away from the pan; no moisture should pool in the pan.
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3
Remove the sautéed mushrooms to a plate. In the same pan, reduce the heat to moderate and add the remaining clarified butter. Add the shallots and sweat, stirring, until soft and translucent but without colour.
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4
Pour in the white wine. Raise the heat and reduce by half; the sharp alcohol edge should be gone and the liquid syrupy at the edges.
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5
Add the demi-glace and the strained cèpe soaking liquid. Stir to combine and bring to a gentle simmer. Cook for 10 minutes; the sauce should coat the back of a spoon and leave a clean line when a finger is drawn through it.
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6
Return all the sautéed mushrooms, including the rehydrated dried cèpes, to the pan. Stir gently and allow the sauce to reheat for 2 to 3 minutes. The sauce should be thick and dark, visibly laden with mushroom pieces, and taste unmistakably of the forest floor — earthy, deep, and complex.
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7
Remove from the heat. Season with salt and white pepper. Fold in the chopped flat-leaf parsley. Serve immediately, garnishing with any reserved whole girolles sautéed separately at the last moment.