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1
Two hours before service, weigh the fish fillets. Apply fine salt at 1% of each fillet's weight — no less. Lay on a rack, uncovered, and refrigerate. This is not a seasoning step alone: the salt draws surface moisture and concentrates flavour throughout the flesh. After two hours, pat the surface dry with kitchen paper. The flesh should feel slightly firmer to the touch.
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2
Fill a deep pot or rice cooker with sufficient water to submerge all four eggs. Bring to exactly 65–68°C and hold that temperature — use a thermometer clipped to the side. Lower the eggs gently into the water and maintain 65–68°C for 45 minutes without interruption. At 65–68°C the white sets to a trembling, barely cohesive gel while the yolk reaches a warm, custard-liquid consistency. Boiling at any point collapses both textures into a conventional hard-boiled egg. This step decides the dish: temperature discipline here is the line between onsen tamago and an ordinary egg.
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3
Cook the rinsed rice according to the absorption method in fresh cold water, using the ratio appropriate to the grain. Keep covered and undisturbed after cooking; the steam-rest period is part of the cooking. The rice is ready when the grains are separate, glossy, and cohesive — they hold their form when pressed but yield without resistance.
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4
Beat the eggs for the tamagoyaki with the cold dashi, usukuchi soy sauce, and mirin until fully combined but not foamy. Heat a rectangular tamagoyaki pan over medium heat and brush lightly with neutral oil. Pour in one-third of the egg mixture; allow it to set at the edges while the centre remains liquid. Roll the egg toward one end of the pan, push the roll back, add the next third of the mixture beneath it, and roll again. Repeat with the remaining third. The finished roll should be golden-yellow, softly set, with clean layered edges visible at the cut face.
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5
Heat a grill or broiler to high. Place the salted salmon fillets skin-side toward the heat source first. Grill without moving until the skin blisters and develops deep caramelised patches — the fat renders visibly and the skin pulls tight. Turn once and finish the flesh side briefly. The fish is done when the flesh flakes at the thickest point under light pressure and the surface carries colour without being dry.
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6
Heat the dashi to just below a simmer. Dissolve the miso paste in a ladleful of the hot dashi before returning it to the pot — this prevents the paste from clumping. Add the silken tofu cubes and rehydrated wakame. Do not allow the soup to boil after miso is added; boiling drives off volatile aromatics and flattens the flavour. The soup must reach the tray at 60°C or above and be drunk immediately. Check temperature with a thermometer or by the fact that steam rises steadily from the bowl surface.
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7
Stir each natto portion vigorously — 50 or more strokes — until the beans are coated in a stretchy, glutinous foam. The texture and rounded flavour develop only through this agitation. Serve immediately with soy sauce alongside.
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8
Arrange each lacquered tray in the ichiju sansai grid: rice bowl at left-front, miso soup at right-front, grilled fish at centre-back. Place tamagoyaki, tsukemono (umeboshi, cucumber or daikon, fukujinzuke), natto, silken tofu with soy sauce, and nori in the remaining positions behind the soup and rice. Crack one onsen tamago from its shell into a small bowl and place on the tray. The architecture is complete when every component is visible and accessible without moving another vessel — the ergonomic logic of centuries of zen arrangement.