
Add the red onion, lime juice, salt, and black pepper to a glass jar or storage container. Cover and stir or shake to combine.
Refrigerate for at least 30 minutes to allow the red onions to cure, stirring or shaking the mixture halfway through so that all the onions are evenly coated with the liquid.
Taste and adjust the salt as needed. Serve immediately or store in the refrigerator for up to 2 weeks.