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1
Remove the speck from refrigeration 30 minutes before slicing and leave it unwrapped at room temperature. The fat should yield perceptibly to light finger pressure before the knife is picked up; cold fat resists cleanly and the meat tears.
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2
Using a long, sharp knife, slice against the grain at a very slight diagonal with a single forward stroke — no back-and-forth sawing. Target 1.5–2 mm thickness. This is where the dish lives or dies: a slice held to the light should be almost translucent yet still carry perceptible body; if it is opaque and waxy it is too thick, if it disintegrates it is too thin.
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3
Lay the slices in a single, loosely overlapping layer on a wooden board. Drizzle with a few drops of aged apple vinegar. The surface of the fat should carry a faint sheen.
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4
Grate fresh horseradish root directly over the board at the moment of service — the volatile heat compounds dissipate rapidly. The grated kren should be pale and moist, not yellowed; a sharp, eye-watering pungency confirms freshness.
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5
Arrange the rye sourdough slices and the Graukäse alongside. Serve within 20 minutes of slicing; beyond that point the exposed fat oxidises and the melting quality on the palate is lost.