1 small red onion
1 jalapeño or serrano chile
2 tablespoons lime juice
1 tablespoon apple cider vinegar or rice vinegar
1/2 teaspoon salt
1/4 teaspoon dried oregano
Cut the onion from stem to root and peel each half. Cut crosswise into thin slices, making a bunch of half-circles.
Mix with remaining ingredients and let sit at room temperature for at least 30 minutes.
If keeping longer than a couple hours, refrigerate. Best eaten within 48 hours but it’s safe for a few days.