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1
Slice the onions strictly lengthwise, following the grain of the bulb from root to tip, producing long intact strands. Set aside. Combine the dashi, soy sauce, mirin, sugar, and sake in a wide, shallow pan and bring to a simmer over medium heat, stirring until the sugar has fully dissolved. The liquid should smell distinctly sweet and savoury — not sharp with raw alcohol.
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2
Add the sliced onions to the simmering sauce. Cook for 4–5 minutes, until the strands have softened and turned translucent but still hold their shape as distinct threads. They should yield to a spoon without collapsing. This is where the dish lives or dies: onions cut across the grain will have disintegrated to mush by this point; onions cut with the grain remain in sweet, cohesive strands that carry both sauce and beef.
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3
Add the thinly sliced beef to the pan in a single layer over the onions. Simmer for 3 minutes only, turning the slices once with tongs or chopsticks. The beef is done when it has just lost its raw colour throughout and each slice is tender, not contracted or curling tightly at the edges. Remove from heat immediately; residual heat will finish the meat.
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4
Assess the sauce: it should coat the back of a spoon with a light, glossy film and pool when tilted — neither thin and watery nor reduced to a sticky glaze. If too thin, return the pan to medium heat for 1–2 minutes without the beef. The correct consistency migrates readily into the rice below once served.
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5
Divide the hot rice among four deep bowls. Ladle the beef and onions over the rice, then spoon the remaining sauce generously over everything (tsuyudaku style). Place a mound of beni shoga to one side of each bowl. Add a raw egg yolk to the centre if using. Serve immediately — the sauce should be actively steaming as it meets the rice.