Special equipment: A heatproof 1-pint jar
Combine the vinegar, 2/3 cup water, onions, salt, peppercorns and coriander in a small pot, bring to a boil, then reduce to a simmer and cook for 5 minutes.
Put the jalapenos in a heatproof 1-pint jar and pour the hot pickling mixture over them. Allow them to sit and soak up the flavor for 30 minutes up to overnight-the longer they sit, the more they will soften and absorb flavor. Stored in the refrigerator, they will keep for up to 1 month.
For a bit less heat, shake some of the seeds out of the jalapenos.
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