If using small onions:
1. In a medium bowl, stir the lime juice, vinegar and salt together.
2. Add the onions to a medium saucepan of boiling water and cook for one minute, or until just softened.
3. Using a slotting spoon, transfer the onions to the bowl with the lime juice mixture.
4. Cover and refrigerate, stirring occasionally, for at least 40 minutes, or until cold.
5. Transfer the mixture to a jar and refrigerate until ready to use.
If using sliced red onion:
1. In a small bowl, combine the sliced onion, lime juice, vinegar and salt.
2. Cover and refrigerate for at least 20 minutes, stirring occasionally.
3. Transfer the mixture to a jar and refrigerate until ready to use.