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Peel the onion, cut in half, and slice thinly.
Pack the onion slices in a 1-pint jar or non-reactive container.
Combine the remaining ingredients in a saucepan and bring to a boil. Stir until the sugar has dissolved.
Pour the hot liquid over the onions in the jar.
Let the onions cool, then cover the jar and refrigerate.
Use on sandwiches, as a condiment, with appetizer spreads, etc.