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In a small bowl, mix together the egg yolk and miso until smooth. Stir in the mirin, sugar, vinegar, and mustard. Refrigerate the dressing until use.
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Have ready a large bowl of ice water. Bring a large pot of generously salted water to a boil. Add the green onions and blanch until tender, 2 minutes. Immediately transfer to the ice water, drain to eliminate all excess liquid, and cut into 1-inch pieces.
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In a serving bowl, toss the dressing, onions, seaweed, and sesame seeds until combined and serve.