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1
Melt half the butter in a wide, heavy-based pan over low heat. Add the sliced onions with a pinch of salt and cook, stirring occasionally, for 30 minutes until deep golden, soft, and fully caramelised. The onions should be the colour of dark amber and smell of sweet, concentrated allium — no raw or sharp edge should remain.
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2
Raise the heat to medium-high. Add the remaining butter. Working in batches, lay the sliced meat into the pan in a single layer and brown on each side. The surface should colour to a deep mahogany crust before the next batch goes in. Do not crowd the pan.
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3
Return all the meat to the pan with the caramelised onions. Pour in the first half of the wine and stir to deglaze, lifting all residue from the base. Add the bay leaf and rosemary. This is where the dish lives or dies: reduce the heat immediately to the barest simmer — the surface should tremble, not boil — and hold it there. A rolling boil will render the meat fibrous and the sauce thin.
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4
After 30 minutes of gentle braising, add the second half of the wine. This second addition preserves the wine's fruit character in the finished sauce. Continue to braise uncovered for a further 1 hour to 1 hour 30 minutes, until the sauce has reduced to a glossy jus that coats the back of a spoon.
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5
Remove and discard the bay leaf and rosemary. Take the pan off the heat. Whisk in a cold knob of butter in small pieces until the sauce turns silky and homogenous. Taste for seasoning. The sauce should be wine-dark, glossy, and coat a spoon cleanly.
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6
Place a slice of dark rye bread on each warmed plate and spoon the carbonnade and its jus directly over and alongside the bread. The bread is integral to the dish, not a garnish — it absorbs the sauce and completes the composition. Serve at once.