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1
Submerge the liver slices in cold milk and leave for 1 hour. The casein binds residual blood and any bitter compounds. Remove, pat thoroughly dry with kitchen paper — a wet surface will not sear.
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2
Place the onions and unsalted butter in a wide, heavy pan over a low flame. Cook slowly, stirring occasionally, for 25 to 30 minutes. The onions are done when they are amber-gold, collapsed, and entirely sweet — rushed onions are burnt and bitter, not the soft caramel required here.
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3
Add the red wine vinegar to the caramelised onions and stir to deglaze, then add the veal jus and reduce briefly until the liquid is nearly absorbed and the onions are glossy. Keep warm.
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4
Season the liver slices with salt and pepper and dredge lightly in flour, shaking off every excess — the coating must be a film, not a crust. Bring a second wide pan to a high heat and add the clarified butter. The butter is ready when it shimmers and the first wisp of smoke appears.
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5
This is where the dish lives or dies. Lay the liver slices in the smoking pan without crowding and sear for exactly 90 seconds per side — no more. The interior must remain rosy pink and yielding at 55–58 °C; above 65 °C the proteins seize, the texture turns grainy, and the flavour shifts from delicate to bitter and metallic. Remove the liver immediately from the pan.
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6
Mound the onions at the centre of each warmed plate. Lay the liver slices over the onions and scatter with chopped parsley. Serve at once — the liver continues to carry over heat and must reach the table without delay.