Bring vinegar, sugar, cumin, and salt to a boil in a 2-quart non-reactive heavy saucepan, then reduce heat and simmer, stirring occasionally, 5 minutes. Add onions and simmer, stirring occasionally, 2 minutes, then transfer to a bowl and cool. Stir in cilantro. Pickled onions can be made 1 week ahead and chilled, covered.
Copyright July 2003, Gourmet magazine. Printed with permission.