Preliminary: Fill a teakettle with 4 cups water and put it up to a boil.
Combine vinegar, 1 cup of water, sugar, salt and peppercorns in a medium sized bowl and stir until sugar is dissolved.
Place the onion slices in a colander in the sink, and slowly pour all the bowling water over them. They will wilt slightly. Drain well, and transfer to the bowlful of marinade.
Cover and allow to marinate, refrigerated or at room temperature, for at least several hours. Store in the refrigerator, and use as needed.
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