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1
Heat the oil in a heavy-based saucepan over low to medium heat. Add the sliced onions with a pinch of salt and cook, stirring regularly, for a minimum of 60 minutes until they are deeply brown, jammy, and reduced to roughly one-quarter of their original volume. They must be a rich chestnut colour throughout — not merely softened or golden.
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2
Add the tomato paste directly to the caramelised onions and stir continuously over medium heat for 2–3 minutes until the paste darkens by one shade and loses its raw, sharp smell.
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3
Pour in the red wine, raise the heat, and reduce by half, stirring to lift any residue from the base of the pan. The sharp alcohol note should give way to a rounder, more integrated fruit aroma.
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4
Add the demi-glace, Worcestershire sauce, and ketchup. Stir to combine and simmer over low heat, uncovered, for 15–20 minutes until the sauce is glossy, deep brown-red, and coats the back of a spoon without running off immediately. Adjust salt and acid balance — the sauce should be intensely savoury with a perceptible but not sharp acidity.
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5
Add the thin-sliced beef directly to the simmering sauce and cook for 2–3 minutes only, stirring once or twice, until the slices are just cooked through with no remaining pink. Remove the pan from the heat immediately — the beef should yield with the lightest pressure and must not be allowed to tighten or shred.
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6
Off the heat, stir in the cold butter in one piece until it melts and is fully absorbed into the sauce, giving it a final gloss. Spoon over mounds of hot steamed rice and serve at once.