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1
Bring a large pot of salted water to a rolling boil; add spaghetti and cook according to package directions until al dente; drain and rinse under cold running water until completely cooled.
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2
Combine mentaiko, soy sauce, mirin, and cold dashi in a large mixing bowl; stir gently with a wooden spoon to break up the roe slightly and distribute evenly — the mixture should remain loose and glossy.
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3
Add the chilled egg yolk to the mentaiko sauce and fold gently to incorporate, being careful not to break the yolk completely — small threads of yolk create a rich, silky emulsion.
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4
Drizzle in cold neutral oil while folding slowly with a rubber spatula until the sauce becomes pale, glossy, and creamy.
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5
Add cooled spaghetti to the mentaiko sauce and toss gently but thoroughly, coating each strand evenly; divide between two chilled bowls.
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6
Garnish each bowl with nori strips, scattered scallions, and a small pinch of shichimi togarashi.
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7
Place a lemon wedge on the rim of each bowl and serve immediately, or chill for up to 2 hours before serving cold.
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8
Toss at the table with lemon juice to taste just before eating.