Make the dressing first by mixing all ingredients in a jar. Place sliced red onion in bottom of serving bowl and drizzle in a couple tablespoons of dressing. Let rest while you prepare the cucumbers and mint.
Peel cucumbers, slice lengthwise, and remove seeds by scraping gently with a spoon. Slice into half-moons and add to serving bowl. Tear mint over top, toss, and add additional dressing by the spoonful. Keep chilled until serving.
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