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1
Lay the masu trout slices in a single layer on a tray. Scatter the 15 g fine sea salt evenly over both surfaces. Leave uncovered for 20–30 minutes — no longer, as the trout's delicate flavour is easily overpowered. Rinse each slice thoroughly under cold running water and pat completely dry with paper towels. The flesh should retain its cherry-blossom pink without appearing blanched or grey.
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2
Steep the kombu piece in the rice vinegar for 15 minutes. Remove the kombu. Dissolve the sugar and salt into the kombu-infused vinegar over low heat, stirring until clear. Allow the sushi-zu to cool to room temperature. Fold it through the warm cooked rice using a cutting motion — do not compress the grains. The rice should be glossy, lightly acidic on the tongue, and hold together when pressed without becoming paste.
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3
Line the base and sides of a round container (a springform cake pan lined with plastic wrap serves as a functional substitute for the traditional circular wooden ohako) with bamboo leaves, overlapping them so that no gap in coverage remains and the leaves extend above the rim. Arrange the cured masu trout slices over the bamboo leaves in overlapping concentric circles, covering the entire surface without gaps. The fat-marbled pink surface must be continuous — this is the presentation face of the finished sushi.
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4
This is where the dish lives or dies: press the kombu-seasoned rice firmly and evenly over the trout in a compact, uniform layer of consistent depth, smoothing the surface level. Lay a final layer of bamboo leaves over the rice to seal. Invert a flat plate or board over the container, then invert the entire assembly so the trout face is now uppermost. Apply firm, even weight — traditional stones or a second container — and press overnight, a minimum of 8–12 hours. Less than 6 hours will produce loose rice that falls apart on slicing; overnight is the standard for structural integrity.
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5
Remove the weight. Before unmoulding, allow the pressed sushi to return to 15–20°C if it has been kept cool — cold rice suppresses flavour. Unmould carefully, retaining the bamboo leaf casing. Using a sharp, wetted knife, slice through the bamboo leaves and the pressed cylinder in one clean cut per wedge, producing the characteristic wedge-shaped portions. The bamboo leaf is retained on each portion as the presentation element. The cut face should reveal clean, unified layers of pink trout over white rice with no crumbling at the edge.