How to Make It
Toss together onion slices and salt in a medium bowl until combined. Transfer onion mixture to a ziplock plastic bag. Add chile, ginger, lemon juice, and vinegar, and seal bag, removing as much air as possible. (If desired, chile and ginger may be wrapped and tied securely in a double layer of cheesecloth before adding to ziplock bag.) Turn bag to coat onions in lemon juice mixture.
Lay ziplock bag flat on a work surface, and let stand at room temperature 2 to 3 hours, turning occasionally. Drain onion mixture; discard chile and ginger. Use onions immediately, or refrigerate in an airtight container until ready to use, up to 2 days.