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1
Bring a large pot of water to a rolling boil. Salt generously. Add the spaghetti and cook until al dente — the pasta must reach full al dente doneness here, as wafu's raw sauce method does not finish-cook the pasta in the sauce. Reserve 120 ml of pasta cooking water before draining.
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2
While the pasta cooks, warm the serving bowls. In each warmed bowl, combine the scraped mentaiko, unsalted butter, cream, light soy sauce, and lemon zest. Work the mixture with a spoon until it forms a uniform, pale-orange raw paste. Do not apply any heat to the bowl. This is the point on which the dish lives or dies: the sauce must remain raw and cool so that the residual heat of the just-drained pasta warms it gently — direct heat destroys the delicate texture and flavour of mentaiko irreversibly.
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3
Drain the pasta immediately and add it hot and undried to the bowl containing the raw sauce. Add two tablespoons of the reserved pasta cooking water. Toss continuously with tongs, working the pasta through the sauce until the butter melts, the cream loosens, and a coherent, lightly clinging sauce coats every strand. The sauce should appear glossy and just barely fluid — not pooling at the base of the bowl.
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4
Add additional pasta water a tablespoon at a time if the sauce tightens before fully emulsifying. Taste and adjust with a few drops of light soy sauce. The balance sought is spicy-creamy-oceanic with clean soy depth — no single element should dominate.
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5
Scatter nori flakes over the pasta immediately before service. Lay katsuobushi flakes across the top last of all, so they meet the heat of the pasta at the table and begin to move. Serve at once, with the flakes visibly dancing — this is both the aromatic finish and the sign that the pasta is hot enough to serve.