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1
Bring a large pot of water to a rolling boil and salt it moderately — mentaiko is already very salty, so restrain the pasta water accordingly. Cook the spaghetti or capellini until al dente according to the packet. Reserve 200 ml of the cooking water before draining. The pasta is ready when it offers faint resistance at the centre when bitten.
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2
While the pasta cooks, combine the scraped mentaiko roe with the cream and butter in a large mixing bowl. Whisk until the roe is evenly distributed and the mixture is smooth. Add the eggs, grated Parmesan, and soy sauce. Whisk to a uniform sauce. The mixture is ready when no streaks of egg remain visible and the roe is fully dispersed.
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3
This is the step on which the dish lives or dies. Remove the bowl from proximity to any direct heat. Add the drained pasta immediately to the egg-and-mentaiko sauce, tossing rapidly so the residual heat of the pasta — and only that heat — cooks the eggs into a glossy, clinging emulsion. Add reserved pasta water a little at a time to loosen as needed. The sauce must coat every strand without a single curd of scrambled egg; if the mixture seizes, stop at once. The finished sauce is fluid, pale gold, and shines against the pasta.
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4
Remove from heat entirely. Add the Japanese mayonnaise and fold it through off the heat to bring the creaminess the Japanese palate requires. The sauce should appear glossy and slightly richer than before the mayonnaise is incorporated.
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5
Divide immediately into four warmed bowls. Lay nori strips and shiso chiffonade across the surface; these raw herbs and aromatics must not cook. The dish is ready when the shiso is vivid green and the nori is still dry and crisp at the moment of service. Serve and eat at once — the sauce stiffens as it cools.