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1
Combine the dashi, soy sauce, mirin, and sugar in a small pan. Stir briefly and bring to a gentle simmer. The sauce is ready when the sugar has fully dissolved and the liquid is uniformly fragrant.
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2
Add the bias-cut chicken thigh to the simmering sauce in a single, loosely spread layer. Simmer over moderate heat until the chicken is barely cooked through — the slices will have just turned opaque with no remaining pink at the centre. Do not overcook; the residual heat of the next stage will carry them forward.
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3
Beat the eggs lightly — they should be combined but not homogeneous; streaks of white and yolk create the layered texture. This is where the dish lives or dies: drizzle roughly two-thirds of the beaten egg around the outer edge of the pan first, allowing those edges to begin to set, then pour the remaining egg in a slow stream into the centre. Place the lid on immediately. Cook for 30–45 seconds under the lid. Remove from heat when the edges are just set and the centre trembles visibly when the pan is moved — the 'yureru' test. The centre must remain liquid-soft.
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4
Divide the hot rice among four bowls. Slide the egg-and-chicken mixture over the rice immediately — delay of even a few seconds allows the egg to continue setting from the pan heat and the rising steam of the rice. The egg should be saucy enough to pool slightly at the base of the bowl.
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5
Lay mitsuba leaves over the surface so they wilt slightly in the steam but retain their green colour. Finish with nori and a light dusting of sansho. Serve at once.