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1
Combine the mentsuyu concentrate and chilled dashi in a ratio of 1:5. Stir briefly and refrigerate until service. The tsuyu must be tasted cold: it should read as just-savoury — appreciably lighter than a standard soba tsuyu. Dilute further if any harshness dominates. Pour into four individual small cups and keep cold.
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2
Bring a large pot of unsalted water to a vigorous, rolling boil. Lower the somen bundles in without breaking them. Cook for 1.5 to 2 minutes only, timing from the moment of immersion. Even thirty seconds beyond the correct point produces a limp, starchy noodle without its characteristic snap; precision here is absolute — this is where the dish lives or dies.
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3
Drain immediately. Transfer the noodles to a colander under cold running water and rub them gently between the hands. Continue rinsing and rubbing until the water runs clear and no surface starch remains. The noodles should feel smooth and distinct — never gummy — and should separate from one another without effort.
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4
Transfer the rinsed noodles to a bowl of ice water and leave briefly until the noodles are thoroughly chilled throughout. Lift in small portions, shake off excess water, and nest each mound on a bed of crushed ice in an individual glass bowl. The noodles should appear translucent white through the ice, with no clumping.
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5
Arrange the condiments — sliced green onion, freshly microplaned ginger, myoga julienne, and toasted sesame — in small separate dishes beside each place setting. Set the cup of chilled mentsuyu alongside. Serve at once: the cold temperature is the preparation's entire premise, and it cannot be recovered once lost.