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1
Melt the butter in a heavy-based saucepan over medium-low heat. Add the sliced onions and cook, stirring at intervals, for 20 minutes or more until deeply golden and thoroughly sweet. The onions must not merely soften — they must caramelize to a rich amber. They are done when they smell sweet and nutty and offer no resistance to the spoon.
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2
Raise the heat to medium-high. Add the beef slices in a single layer, in batches if necessary, and sear briefly until no pink remains on the surface. Set the beef aside. Add the button mushrooms to the pan and cook until their moisture has released and evaporated. They are done when the pan is dry and the mushrooms are lightly colored.
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3
Pour in the red wine and reduce by half over medium-high heat, scraping up any residue from the base of the pan. This is where the dish lives or dies: an insufficient reduction at this stage denies the sauce its characteristic wine depth and dark gloss — the wine must be concentrated, not merely added. The reduction is done when the volume is visibly halved and the aroma shifts from sharp to round.
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4
Stir in the tomato paste and cook for two minutes, pressing it against the base of the pan until it deepens in color. Add the demi-glace and sugar. Return the seared beef to the pan. Stir to combine and bring to a gentle simmer.
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5
Simmer uncovered for a minimum of 30 minutes, stirring occasionally, until the sauce is glossy, dark, and coats the back of a spoon. The sauce is done when a line drawn through the coating on the spoon holds its edges cleanly.
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6
Remove from heat. Add the dark chocolate and stir gently until it dissolves entirely into the sauce, lending roundness without sweetness. Taste and adjust sugar if required. The sauce is ready when it is glossy, wine-dark, and uniformly smooth.
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7
Serve the sauce over mounded Japanese short-grain rice. The rice should be freshly cooked by the absorption method and retained in a warm state. The sauce is ladled generously so that it pools at the base of the rice mound.