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1
Combine the dashi, soy sauce, mirin, sake and sugar in a wide, shallow saucepan. Bring to a gentle simmer over medium heat, stirring until the sugar is fully dissolved. Taste: the sauce should be distinctly sweet-savoury — sweeter than standard Japanese simmered preparations. Adjust sugar accordingly.
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2
Add the sliced onions to the simmering tsuyu. Cook over medium heat, turning occasionally, until the onions are completely translucent and have released all their sweetness into the sauce — approximately 8–10 minutes. The onions must reach full translucency; semi-cooked onion provides neither the structural sweetness nor the soft texture the dish requires. Confirm by pressing a slice: it should offer no resistance.
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3
This is where the dish lives or dies. Lay the 2 mm beef slices into the pan in a single layer over the onions. Simmer over medium heat for 1–2 minutes only, turning the slices once. The beef is done the moment it transitions from pink to just-cooked grey — juicy, not dried. Any further cooking will expel moisture and toughen the fibres beyond recovery; the onions, not the beef, have built the sauce's flavour. Remove from heat immediately.
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4
If the sauce has thickened and begun to cling dry around the meat, add a small ladleful of warm dashi to loosen it to a fluid, glossy consistency that coats the beef lightly without pooling. The sauce should flow when the pan is tilted.
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5
Divide the hot rice among four deep bowls. Spoon the beef, onions and sauce evenly over the rice, ensuring each portion receives a generous covering of sauce. Place a raw egg yolk at the centre of each bowl and arrange a mound of drained beni shoga to one side. Serve immediately.