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1
Combine the cold dashi and cold concentrated mentsuyu in a ratio of approximately 6:1. Stir briefly to integrate. Transfer to the refrigerator or set over ice. The tsuyu must be thoroughly cold before service — it should feel sharply cold to the fingertip.
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2
Prepare the condiments: slice the ginger to extreme thinness or grate finely; slice the myōga and negi finely. Arrange each in a separate small dish. Set aside at room temperature until service.
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3
Bring at least 4 litres of water to a full, vigorous boil in the largest available pot. Lower the sōmen into the water. Cook for 90 to 120 seconds only — the noodles are done when they have lost all opacity and yield at once to the bite without any raw resistance at the centre. Thirty seconds beyond this point is already too long.
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4
This is where the dish lives or dies. Drain the noodles immediately and transfer to a large bowl under a cold running tap. Rub the noodles vigorously between both palms while the water runs over them — work continuously until the water running off is completely clear and the noodle surface feels slippery rather than starchy. A starchy surface will cause the noodles to clump at once and forfeit the characteristic slippery texture that defines cold sōmen.
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5
Arrange the rinsed noodles in loose coils on a bed of crushed ice or immerse them in ice water in a bowl or deep dish. The noodles should be visibly cold — glistening white against the ice. Room-temperature service defeats the purpose of the dish entirely.
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6
Pour the cold tsuyu into individual dipping cups. Set the condiment dishes alongside. Each diner adds ginger, myōga, and negi to the tsuyu to their preference, lifts a small tangle of noodles with chopsticks, dips briefly, and eats. The noodle should almost disappear on the tongue — the eating experience is primarily textural and thermal.