Blanching the onion and celery mellows their flavors, so they don't overpower the edamame and tangy vinaigrette.
Combine first 3 ingredients in a large bowl, stirring with a whisk. Add oil, stirring with a whisk until well combined.
Cook edamame in boiling water 4 minutes. Add onion and celery; cook 1 minute. Drain well. Add edamame mixture to vinaigrette; toss well to coat. Stir in parsley. Chill 1 hour.
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