Prep the Onion: Peel and thinly slice the red onion into half-moons or rings.
Pickle: Place the sliced onion in a jar or bowl. Pour the lime juice over the onions and add a generous pinch of salt. Toss to combine, making sure the onions are fully coated in the juice.
Marinate: Let the onions sit at room temperature for at least 5 minutes, stirring once or twice. They’ll soften and turn bright pink as they pickle. For deeper flavor, refrigerate for a few hours or overnight.
Serve: Use immediately or store in the fridge for up to 1 week. These are perfect on tacos, grain bowls, sandwiches, and more.