To make the onion spicier, reserve and add the chile seeds with the vinegar. The sweet and spicy pickled onions can be refrigerated in an airtight container for up to 1 week.
Place onions in medium heatproof bowl. Bring vinegar, sugar, jalapeños, and salt to simmer in small saucepan over medium-high heat, stirring occasionally, until sugar dissolves. Pour vinegar mixture over onions, cover loosely, and let cool completely, about 30 minutes. Once cool, drain and discard liquid.
