Combine sliced onions and salt in a medium-sized bowl; cover and chill for 1 1/2 hours.  Transfer the sliced onions to a colander; rinse under cold water and drain.  Return the rinsed onions to the bowl.

In a small saucepan, combine the sugar and remaining ingredients. Bring to a simmer over medium heat, stirring until the sugar dissolves. Pour the hot vinegar mixture over the onions. Gently mix and let stand at room temperature for 1 hour.

Cover and refrigerate for 24 hours before serving. Store in an airtight container in the refrigerator for up to 2 weeks.