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1
Remove the seared steaks from the pan and set aside to rest. Pour off all but a thin film of fat, leaving the fond intact on the base of the pan.
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2
Return the pan to high heat. Add the mortar-cracked peppercorns and toast them briefly in the residual fat, stirring once, until they are fragrant and beginning to colour. The irregular crack — coarse shards and fine dust together — is the architecture of heat distribution in the finished sauce; too fine and the sauce becomes uniformly hot, too coarse and individual fragments deliver aggressive, uneven heat.
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3
Add the cognac. Tilt the pan to ignite or apply a flame directly. Allow the cognac to flambé fully: the flame burns off the raw alcohol while caramelising the residual sugars in the fond. This is where the dish lives or dies — a pan not hot enough will not ignite; if the flame does not catch, remove the pan from the heat, return it to high heat, and introduce the cognac again. The sauce is ready to proceed only once the flame has died completely of its own accord.
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4
Add the stock and reduce over high heat by two-thirds, until the liquid is dark, glossy, and visibly syrupy around the edges of the pan.
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5
Pour in the heavy cream and reduce over medium-high heat, stirring occasionally, until the sauce reaches a nappante consistency — it coats the back of a spoon cleanly and holds a line drawn through it with a finger.
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6
Remove the pan from the heat. Add the cold butter and swirl or stir continuously until it is fully absorbed and the surface carries a distinct, even sheen.
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7
Season with fleur de sel. The finished sauce is beige-gold in colour, glossy, with visible pepper fragments suspended throughout. Spoon beside — not over — the steak immediately.